The website service I use is Boomspeed.com They have (or did) a free account that you could set up to for such needs.
But, if you have the picture (or pictures) that you want to use, then you just have to put all the components of your sig together.
If you have any questions, send me a PM and I'll certainly try to help. Another good person to ask would be my buddy Chuut. Many sigs that you see people use have been created by him. He just seems to have the uncanny ability to find the best pics for whatever the need. I even use a lot of the same search programs he does to find good pics, but can't seem to find the same frickin ones at times.
Anyway, hope this helps.
Agave