To make a jpeg on your comp into a signature you will first have to host it somewhere on the net. There are some good free hosting sites, I think many people use
http://yoursighost.com/Or if you see a pic that you like on the net, right click it, select properties, where it says \'Address(URL):\' copy the address given.
Now open your Planeshift profile, select \'edit signature\'. Click on the IMG button and put the URL address where your sig is now located into the box that has appeared. This should give the image you wanted as your sig.
Hope this helps