First off, go to forum feedback and there is a long thread for signatures.The way you do it is attach it to a post you do(when you are in the preview/edit screen you will see attach file.Then after you post it, go to that post and put your cursor over the 'attachment' link.On the bottom of your browser you should see something like http://www.tractorbynet.com/forumfiles/18-xxxxx-xxx.gif.(write that down),that is what you put in your profile with image tags under signature and then you should see it next time you post.
P.S. Muhammad wants it to be under 4K in size(smaller is better though), and 65 pixels wide by 75 pixels high.