I have a picture saved on my computer, but how do I transfer it over to my sig everytime I post?

 

Very carefully.

 

I don't think you can. You have to either host it here if you are a member or you can host it at another site and link it in using IMG tags. I'm confident that Admins will chime in if I am wrong or there is something more to add.

 

Signup for an S2KI membership. Then you can upload your image to your s2ki photo gallery and include it in your signature