That's interesting... I'm not sure why that would happen to you, but are you sure you're using the right codes to post it?
Be sure to upload your picture to an image sharing site (such as photobucket, but since you have extra stuffies in your siggie, I recommend Tinypic). Then, copy+paste the IMG code into your signature, and you shouldn't be getting any of those issues.
Edit--
If you have already done this, it couldn't hurt to try uploading it again, in case something derped up in the link.
Or, try removing some of the extras from your signature. See what you can do to just get the image working, then try adding extras.