Most forums you will come across enable their members to add a signature to their posts. They contain whatever information you would like to include such as pictures, quotes, links to other websites, etc. Here on the Feral-heart forums, you might find that a few members provide their in-game username as well as a drawing / fan art of their character(s). Jenga already explained the usage of pictures and that all content you would like to include should be appropriate. With this in mind, you are free to get creative with your signature. c: