Go to your control panel, then edit signature, and hover over some of the buttons to see what they do. The insert picture one is shaped like a little mountain, but you need to have the URL for the picture as well (right click then copy image location, and then hit ctrl+v to copy it into the box that pops up for your sig)
you need to get the picture hosted someplace to begin with. You can upload it to your gallery on the forums here or you can host it someplace else such as www.photobucket.com or http://imageshack.us . After that get the url for it go into the drop down menu where it says "user options" and choose "your profile". From there on the left you'll see "Edit Signature". Click that and put the URL of your image between . actually, this link should take you to the page where you edit your Sig: http://hipforums.com/forums/profile.php?do=editsignature