Save as .jpg
go to shop
textures
upload it.

I recommend that you save it as either .png, .gif or .tga.
Why? Because .jpg most of the times lowers the quality of the pic, thus making it look worse.
There isn't any kind of quality loss in .png, .gif and .tga.

Just a small advice ;o

Also, please don't bump old threads for your own questions. Instead, you could've either searched for your answer (which would've been better), or made your own thread for the question.

Closing this thread. Let this post be a warning for you.