Same way you post other photos but go to "User CP" at the top and paste it into your sig.
You need to find a web hosting service eg: http://photobucket.com. This will give your pictures their own individual URLs.
When your photos have a URL
1. get the photo to the size you want by double left-clicking on it to make it big
2. rightclick on the photo you want to use
3. properties
4. rightclick on URL address
5. select all
6. rightclick again
7. copy
8. go back to the text where you want to put the photo
9. type: