The easiest way (for me) to do a signature is to upload a picture into one of your album on here. Then click on the picture (make it big), directly copy it (Ctrl + C). Then go to User CP, Edit Signature. You will see a big blank box, where you "could" write something. Click on that as if you are getting ready to type, but PASTE instead (Ctrl + V)
Your picture should then show up. Don't forget to SAVE.