Ok, I've found a bit more information on this - not sure if it will clarify at all. When I added the img tag and opened the jsp page it was actually running my java code with an empty request. Naturally, since it was a totally empty request, there were no parameters and hence the NullPointer. So, i went into my java and put a catch for that.
Now there's no null pointer, but there's also no image and no error message to give me any clue about why.
Any ideas? This one's makin' me crazy.