What it probably means is that the picture size is too big (not dimentions, but bytes) - you need to save it as web ready, which isn't as good of quality. Which image editing program are you using?
I'm horrible at explaining this stuff (obviously!) But I can give you my version of instructions if I know the program you're using
I make mine 75x75 pixels, and save as "medium" quality in photoshop. That seems to get them down far enough to be able to use as an avatar.