This is common knowledge for most of us, and again, this is for the choobs in this section who request a signature but cannot use it.
Part 1. Get the image URL by copying the link location, not the picture itself.
Part 2. Go into your user Control Panel, And under Settings and Options, click Edit Signature. Shouldn't be that hard, really.
Part 3. With your image URL, you can do one of two things. either put [IMG] and [/IMG] around the link to make it an image, or click on the Insert Image button in the toolbar, and ctrl+v your image link.
Part 4. You're done already. But If you want to double check, click preview signature, and it should come out on the display above your signature editing box.
Part 5. Save it.
There's no need to thank me, again, just some things newbies need to know. And this is so they stop persistently putting up threads, complaining that their signature isn't working. Domo Arigatou.