Here's a fun bug.  I have a script that loads a JPEG with imagecreatefromjpeg and then sends it directly to the browser.  No error from PHP, and it really was returning a resource, but when I called imagejpeg, it complained about an invalid resource.  It took me about 15 minutes to realize that while the image was there, it had been corrupted by being transfered in ftp's ASCII mode.

I don't know how practical it is because I haven't looked at the code, but it would be nice for imagecreatefromjpeg to return an error when the JPEG is corrupt.  Likewise with the rest of the imagecreatefrom_ family.