I thought I'd chime in here - for no other reason than it seems like I probably should sooner or later.
I don't much like the idea of having requirements on posting images - except for size - for the simple reason I want anyone to post whatever they can and want to. If someone wants to fool with thumbnails - great - but I don't want to "require" it simply because some people might not want to bother preparing two separate images for posting.
As for having the script handle the resizing, I don't think that's an option.
Don