How do I get a account signature??

I see many people on the forums that have their own signature under their text that they have written. For example under this one. I have a good picture that I want to use as a signature but when I go to my profile and then edit signature. When I try to paste the picture or paste the website code the website says IMG files isn't allowed! How can I have a picture?? I have watched movies but noting...:(
Please help me