to include a pic in your signature file [found under your profile settings] it will show up with each post but you need to upload it to a hosting site first, like photobucket.
u can attach photos to your actual post, but u have to do this each time, these photos can come right from your hard drive, no need to have them hosted on another site, look at the bottom of the post window, u will see [additional options] click on it to open the menu, choose [browse] and find the photo u want to include in ur post