Is it .png, .gif, .jpg, what? All images have a tag. When I have that problem, it's usually because something is saved as a .gif and it's not a .gif. Like...if the icons in my sig were saved as .png, that would be invalid because they're moving, which moving images are .gif images.
I've had this problem before. It should be able to be solved by changing the file extension.
Techy people, feel free to do a better job of explaining btw. I kinda suck at this. lol