Support Signature Images Not Working

Everytime I try to put a different image in my signature, it gives me this:




And, yes, I've tried many different images, but none work.
I'm trying to put this in it(yes, the dimensions are the right size):

How do I fix this?

 

Nope, still didn't work.

 

I know, I try to remove the current one and replace it with that other one, but it still says that. It says it no matter what images I use, even if I just try to re-save my current signature without changing the image it still gives me the error.