Take the image you want and host it at photobucket.com or some other image hosting site. Copy the URL of the image, use the image button on the signature editing page, and paste it. It'll be in IMG tags so it shows up as an image to the forum. (Photobucket has it already in IMG tags so it'll be easier. )
Make sure the image is the appropriate size, however. Check the 'Signature Size Restrictions' thread to make sure. If you're confused about something, feel free to ask.