You do it just as if you are posting a photo in the body of the message only in the signature line in your profile.
The photo will have to be hosted either here or on a photo hosting site. I use Geocities.
Go to your profile and choose the signature tab. Put the link of the photo in the signature line.
If you want to host the photo here it is the button that is an image with a disk over it. If you drag your mouse across it and it should say host an image and it is usually right next to the button that says insert image.