Symphony.

There is a misconception between file size and screen size with images when it comes to image manipulation. Even for a small image, if the total pixel surface area is large, the amount of memory required is significantly large. In this instance, checking by file size does not guarantee that manipulation via GD will succeed.

There should be a configuration/define to set the allowable maximum image size for upload.

Should this be a maximum that users can upload or a maximum over which JIT will refuse to process when asked to? Perhaps an optional numeric setting in System > Preferences such as:

Do not process images larger than [ 4 ] MB in size

Or simply a default in the config.php.

Will also need some sensible fallback image too, for a) when images are not found and a 404 is returned or b) JIT refuses to process (401, 500?). Could this also be defined in the config globally, or a per-URL basis (like Gravatar).