If it's a screenshot from FH (like mine) then find a website like TinyPic, and put the screenshot pic into the place it tells you to on the website.
It should then upload it once you've pressed the upload button and then copy the "IMG Code for Forums & Message Boards" into the signature box and once you have done that, press change profile at the bottom of the page.
The same thing goes if it's a pic from the internet, but you should save that to "pictures" in the "Library" in your documents and then use the same technique.