diff --git a/pixstrip-core/src/error.rs b/pixstrip-core/src/error.rs
index 6d278a7..1881db4 100644
--- a/pixstrip-core/src/error.rs
+++ b/pixstrip-core/src/error.rs
@@ -1 +1,27 @@
-// Error types
+use std::path::PathBuf;
+
+#[derive(Debug, thiserror::Error)]
+pub enum PixstripError {
+    #[error("Failed to load image '{path}': {reason}")]
+    ImageLoad { path: PathBuf, reason: String },
+
+    #[error("Unsupported format: {format}")]
+    UnsupportedFormat { format: String },
+
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("Output file already exists: {path}")]
+    OutputExists { path: PathBuf },
+
+    #[error("Processing error in '{operation}': {reason}")]
+    Processing { operation: String, reason: String },
+
+    #[error("Preset error: {0}")]
+    Preset(String),
+
+    #[error("Configuration error: {0}")]
+    Config(String),
+}
+
+pub type Result<T> = std::result::Result<T, PixstripError>;
diff --git a/pixstrip-core/src/types.rs b/pixstrip-core/src/types.rs
index 2fe9693..f512637 100644
--- a/pixstrip-core/src/types.rs
+++ b/pixstrip-core/src/types.rs
@@ -1 +1,147 @@
-// Core domain types for image processing
+use std::fmt;
+use std::path::{Path, PathBuf};
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum ImageFormat {
+    Jpeg,
+    Png,
+    WebP,
+    Avif,
+    Gif,
+    Tiff,
+}
+
+impl ImageFormat {
+    pub fn from_extension(ext: &str) -> Option<Self> {
+        match ext.to_lowercase().as_str() {
+            "jpg" | "jpeg" => Some(Self::Jpeg),
+            "png" => Some(Self::Png),
+            "webp" => Some(Self::WebP),
+            "avif" => Some(Self::Avif),
+            "gif" => Some(Self::Gif),
+            "tiff" | "tif" => Some(Self::Tiff),
+            _ => None,
+        }
+    }
+
+    pub fn extension(&self) -> &'static str {
+        match self {
+            Self::Jpeg => "jpg",
+            Self::Png => "png",
+            Self::WebP => "webp",
+            Self::Avif => "avif",
+            Self::Gif => "gif",
+            Self::Tiff => "tiff",
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ImageSource {
+    pub path: PathBuf,
+    pub original_format: Option<ImageFormat>,
+}
+
+impl ImageSource {
+    pub fn from_path(path: impl AsRef<Path>) -> Self {
+        let path = path.as_ref().to_path_buf();
+        let original_format = path
+            .extension()
+            .and_then(|ext| ext.to_str())
+            .and_then(ImageFormat::from_extension);
+        Self {
+            path,
+            original_format,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub struct Dimensions {
+    pub width: u32,
+    pub height: u32,
+}
+
+impl Dimensions {
+    pub fn aspect_ratio(&self) -> f64 {
+        self.width as f64 / self.height as f64
+    }
+
+    pub fn fit_within(self, max: Dimensions, allow_upscale: bool) -> Dimensions {
+        if !allow_upscale && self.width <= max.width && self.height <= max.height {
+            return self;
+        }
+
+        let scale_w = max.width as f64 / self.width as f64;
+        let scale_h = max.height as f64 / self.height as f64;
+        let scale = scale_w.min(scale_h);
+
+        if !allow_upscale && scale >= 1.0 {
+            return self;
+        }
+
+        Dimensions {
+            width: (self.width as f64 * scale).round() as u32,
+            height: (self.height as f64 * scale).round() as u32,
+        }
+    }
+}
+
+impl fmt::Display for Dimensions {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}x{}", self.width, self.height)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum QualityPreset {
+    Maximum,
+    High,
+    Medium,
+    Low,
+    WebOptimized,
+}
+
+impl QualityPreset {
+    pub fn jpeg_quality(&self) -> u8 {
+        match self {
+            Self::Maximum => 95,
+            Self::High => 85,
+            Self::Medium => 72,
+            Self::Low => 55,
+            Self::WebOptimized => 65,
+        }
+    }
+
+    pub fn png_level(&self) -> u8 {
+        match self {
+            Self::Maximum => 2,
+            Self::High => 3,
+            Self::Medium => 4,
+            Self::Low => 6,
+            Self::WebOptimized => 5,
+        }
+    }
+
+    pub fn webp_quality(&self) -> f32 {
+        match self {
+            Self::Maximum => 95.0,
+            Self::High => 85.0,
+            Self::Medium => 72.0,
+            Self::Low => 50.0,
+            Self::WebOptimized => 75.0,
+        }
+    }
+
+    pub fn label(&self) -> &'static str {
+        match self {
+            Self::Maximum => "Maximum",
+            Self::High => "High",
+            Self::Medium => "Medium",
+            Self::Low => "Low",
+            Self::WebOptimized => "Web-optimized",
+        }
+    }
+}
diff --git a/pixstrip-core/tests/error_tests.rs b/pixstrip-core/tests/error_tests.rs
new file mode 100644
index 0000000..04e7779
--- /dev/null
+++ b/pixstrip-core/tests/error_tests.rs
@@ -0,0 +1,46 @@
+use pixstrip_core::error::PixstripError;
+
+#[test]
+fn error_display_image_load() {
+    let err = PixstripError::ImageLoad {
+        path: "/tmp/bad.jpg".into(),
+        reason: "corrupt header".into(),
+    };
+    let msg = err.to_string();
+    assert!(msg.contains("/tmp/bad.jpg"));
+    assert!(msg.contains("corrupt header"));
+}
+
+#[test]
+fn error_display_unsupported_format() {
+    let err = PixstripError::UnsupportedFormat {
+        format: "bmp".into(),
+    };
+    assert!(err.to_string().contains("bmp"));
+}
+
+#[test]
+fn error_display_io() {
+    let io_err = std::io::Error::new(std::io::ErrorKind::NotFound, "file missing");
+    let err = PixstripError::Io(io_err);
+    assert!(err.to_string().contains("file missing"));
+}
+
+#[test]
+fn error_display_output_exists() {
+    let err = PixstripError::OutputExists {
+        path: "/tmp/out.jpg".into(),
+    };
+    assert!(err.to_string().contains("/tmp/out.jpg"));
+}
+
+#[test]
+fn error_display_processing() {
+    let err = PixstripError::Processing {
+        operation: "resize".into(),
+        reason: "invalid dimensions".into(),
+    };
+    let msg = err.to_string();
+    assert!(msg.contains("resize"));
+    assert!(msg.contains("invalid dimensions"));
+}
diff --git a/pixstrip-core/tests/types_tests.rs b/pixstrip-core/tests/types_tests.rs
new file mode 100644
index 0000000..5ef828b
--- /dev/null
+++ b/pixstrip-core/tests/types_tests.rs
@@ -0,0 +1,83 @@
+use pixstrip_core::types::*;
+use std::path::PathBuf;
+
+#[test]
+fn image_format_from_extension() {
+    assert_eq!(ImageFormat::from_extension("jpg"), Some(ImageFormat::Jpeg));
+    assert_eq!(ImageFormat::from_extension("JPEG"), Some(ImageFormat::Jpeg));
+    assert_eq!(ImageFormat::from_extension("png"), Some(ImageFormat::Png));
+    assert_eq!(ImageFormat::from_extension("webp"), Some(ImageFormat::WebP));
+    assert_eq!(ImageFormat::from_extension("avif"), Some(ImageFormat::Avif));
+    assert_eq!(ImageFormat::from_extension("gif"), Some(ImageFormat::Gif));
+    assert_eq!(ImageFormat::from_extension("tiff"), Some(ImageFormat::Tiff));
+    assert_eq!(ImageFormat::from_extension("tif"), Some(ImageFormat::Tiff));
+    assert_eq!(ImageFormat::from_extension("xyz"), None);
+}
+
+#[test]
+fn image_format_extension_string() {
+    assert_eq!(ImageFormat::Jpeg.extension(), "jpg");
+    assert_eq!(ImageFormat::Png.extension(), "png");
+    assert_eq!(ImageFormat::WebP.extension(), "webp");
+    assert_eq!(ImageFormat::Avif.extension(), "avif");
+}
+
+#[test]
+fn image_source_from_path() {
+    let source = ImageSource::from_path("/tmp/photo.jpg");
+    assert_eq!(source.path, PathBuf::from("/tmp/photo.jpg"));
+    assert_eq!(source.original_format, Some(ImageFormat::Jpeg));
+}
+
+#[test]
+fn image_source_unknown_format() {
+    let source = ImageSource::from_path("/tmp/file.xyz");
+    assert_eq!(source.original_format, None);
+}
+
+#[test]
+fn dimensions_display() {
+    let dims = Dimensions { width: 1920, height: 1080 };
+    assert_eq!(dims.to_string(), "1920x1080");
+}
+
+#[test]
+fn dimensions_aspect_ratio() {
+    let dims = Dimensions { width: 1920, height: 1080 };
+    let ratio = dims.aspect_ratio();
+    assert!((ratio - 16.0 / 9.0).abs() < 0.01);
+}
+
+#[test]
+fn dimensions_fit_within() {
+    let original = Dimensions { width: 4000, height: 3000 };
+    let max_box = Dimensions { width: 1200, height: 1200 };
+    let fitted = original.fit_within(max_box, false);
+    assert_eq!(fitted.width, 1200);
+    assert_eq!(fitted.height, 900);
+}
+
+#[test]
+fn dimensions_fit_within_no_upscale() {
+    let original = Dimensions { width: 800, height: 600 };
+    let max_box = Dimensions { width: 1200, height: 1200 };
+    let fitted = original.fit_within(max_box, false);
+    assert_eq!(fitted.width, 800);
+    assert_eq!(fitted.height, 600);
+}
+
+#[test]
+fn dimensions_fit_within_allow_upscale() {
+    let original = Dimensions { width: 800, height: 600 };
+    let max_box = Dimensions { width: 1200, height: 1200 };
+    let fitted = original.fit_within(max_box, true);
+    assert_eq!(fitted.width, 1200);
+    assert_eq!(fitted.height, 900);
+}
+
+#[test]
+fn quality_preset_values() {
+    assert!(QualityPreset::Maximum.jpeg_quality() > QualityPreset::High.jpeg_quality());
+    assert!(QualityPreset::High.jpeg_quality() > QualityPreset::Medium.jpeg_quality());
+    assert!(QualityPreset::Medium.jpeg_quality() > QualityPreset::Low.jpeg_quality());
+}