And you're using the correct link, right? Not a thumbnail version or something?
I've seen this happen to other people and I think they had to contact an admin about it. Not sure if they were doing something wrong or if it was just a glitch. You can try using the image URL from my siggy and putting it in yours just to see if it's your account or if it's the specific image you're trying to use.
To clarify, you are using the direct link to the image between [IMG][/IMG] tags like these, right? And you're placing it in the text box under "Edit Signature" and NOT the URL spot for "Signature Picture," correct? If you're doing all that, then it definitely must be some glitch.