diff --git a/apps/ml-service/src/image.rs b/apps/ml-service/src/image.rs
new file mode 100644
index 0000000..dca6eec
--- /dev/null
+++ b/apps/ml-service/src/image.rs
@@ -0,0 +1,121 @@
+use crate::error::ServiceError;
+use image::ImageReader;
+use ndarray::Array4;
+use std::io::Cursor;
+
+/// Preprocesses an image from bytes into an NHWC f32 array.
+///
+/// # Arguments
+/// * `bytes` - Raw image bytes (PNG, JPEG, etc.)
+/// * `input_size` - Target size for both width and height
+///
+/// # Returns
+/// * `Result<Array4<f32>, ServiceError>` - Array with shape (1, input_size, input_size, 3) in NHWC format
+///   or BadRequest error if the image is invalid
+pub fn preprocess_image(bytes: &[u8], input_size: u32) -> Result<Array4<f32>, ServiceError> {
+    // Decode image from bytes
+    let cursor = Cursor::new(bytes);
+    let reader = ImageReader::new(cursor)
+        .map_err(|_| ServiceError::BadRequest("Uploaded file is not a valid image".to_string()))?;
+
+    let image = reader
+        .decode()
+        .map_err(|_| ServiceError::BadRequest("Uploaded file is not a valid image".to_string()))?;
+
+    // Convert to RGB
+    let rgb_image = image.to_rgb8();
+
+    // Resize to input_size x input_size using Triangle filter
+    let resized = image::imageops::resize(
+        &rgb_image,
+        input_size,
+        input_size,
+        image::imageops::FilterType::Triangle,
+    );
+
+    // Create Array4<f32> with shape (1, input_size, input_size, 3) in NHWC format
+    let (width, height) = resized.dimensions();
+    let pixels = resized.into_raw();
+
+    // pixels is a flat Vec<u8> in RGB order (R, G, B, R, G, B, ...)
+    // Convert to f32 and reshape to (1, height, width, 3)
+    let float_pixels: Vec<f32> = pixels.iter().map(|&p| p as f32).collect();
+
+    let array = Array4::from_shape_vec(
+        (1, height as usize, width as usize, 3),
+        float_pixels,
+    )
+    .map_err(|_| {
+        ServiceError::BadRequest("Failed to reshape image data".to_string())
+    })?;
+
+    Ok(array)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Helper to create a simple 2x1 RGB PNG with specific pixel values
+    fn create_test_png() -> Vec<u8> {
+        use image::RgbImage;
+
+        // Create a 2x1 image
+        let mut img = RgbImage::new(2, 1);
+
+        // Set first pixel to [10, 20, 30]
+        img.put_pixel(0, 0, image::Rgb([10, 20, 30]));
+
+        // Set second pixel to [40, 50, 60]
+        img.put_pixel(1, 0, image::Rgb([40, 50, 60]));
+
+        // Encode to PNG bytes
+        let mut png_bytes = Vec::new();
+        img.write_to(&mut Cursor::new(&mut png_bytes), image::ImageFormat::Png)
+            .expect("failed to encode PNG");
+
+        png_bytes
+    }
+
+    #[test]
+    fn preprocess_image_correct_shape_and_values() {
+        let png_bytes = create_test_png();
+        let result = preprocess_image(&png_bytes, 2);
+
+        assert!(result.is_ok(), "preprocessing should succeed");
+        let array = result.unwrap();
+
+        // Check shape is (1, 2, 2, 3)
+        assert_eq!(array.shape(), &[1, 2, 2, 3]);
+
+        // After resizing 2x1 to 2x2, we expect interpolation
+        // Check that first pixel channel values are present (at least the first row)
+        // The exact values depend on interpolation, but we can verify the structure
+        let first_batch = &array.slice(ndarray::s![0, .., .., ..]);
+        assert_eq!(first_batch.shape(), &[2, 2, 3]);
+
+        // Verify first pixel (0, 0) has 3 channels
+        let pixel_0_0 = &first_batch.slice(ndarray::s![0, 0, ..]);
+        assert_eq!(pixel_0_0.len(), 3);
+
+        // The first pixel should be close to [10, 20, 30] (may be interpolated)
+        // We'll just verify it's in a reasonable range
+        assert!(pixel_0_0[0] >= 5.0 && pixel_0_0[0] <= 25.0);
+        assert!(pixel_0_0[1] >= 15.0 && pixel_0_0[1] <= 35.0);
+        assert!(pixel_0_0[2] >= 25.0 && pixel_0_0[2] <= 45.0);
+    }
+
+    #[test]
+    fn invalid_image_returns_bad_request() {
+        let invalid_bytes = b"not a valid image";
+        let result = preprocess_image(invalid_bytes, 224);
+
+        assert!(result.is_err());
+        match result.unwrap_err() {
+            ServiceError::BadRequest(detail) => {
+                assert_eq!(detail, "Uploaded file is not a valid image");
+            }
+            _ => panic!("expected BadRequest error"),
+        }
+    }
+}
diff --git a/apps/ml-service/src/main.rs b/apps/ml-service/src/main.rs
index cbff549..54150ce 100644
--- a/apps/ml-service/src/main.rs
+++ b/apps/ml-service/src/main.rs
@@ -1,5 +1,6 @@
 mod config;
 mod error;
+mod image;
 mod routes;
 
 fn main() {