Prevent unnecesary rebuilding of derivative images

Would it be possible to add a check in image.module to use existing derivative image when the settings change won't result in a different sized image?

Example: I change the Thumbnail Size. Preview size is the same. But now, when I go into any image gallery, all derivatives - thumbnail as well as preview - are deleted, and re-generated. In all cases, the preview image was not changed - so it was not necessary to delete it and regenerate it.
Since shared host providers have a huge problem with the script times, I always get timeouts and Internal Server Error messages in such cases.
So, asking if the module can determine the new derivative size, compare with existing derivative image, and if they are the same, then skip calling the scale or scale/crop routines.