When working with the image field in eZ Platform 1.4.x uploading an invalid image is possible and is not displayed as an error - just a failed preview.
1. Rename a file to .jpg (not a JPEG)
2. Upload file to image field
3. Observe broken image preview
4. Publish and see the error (see attachment)
This is already an improvement over eZ Publish where you can upload an invalid image, but conversion to image aliases is attempted regardless of whether it is a valid image (see https://jira.ez.no/browse/CS-5073).
as the invalid image is not being converted to a smaller size