Please do not use signature images out side the signature area in your profile.
If the remote server deletes these images, they get broken on the forums, you can not automatically go and fix them. Each message has to be hand edited to fix it. When you use them in your profiles signature box these can be updated and it changes every message that you have ever posted on the site with one change.
Also on these images, do not use more than 3 to 5, they should be no larger than 150x150 pixels in size. If you have more signature images than post content there's something wrong somewhere :P
Thanks!