ok, you upload the picture to imageshack or photobucker (any picture website really)
then you copy the IMG code (i think photobucket only) and paste it into your shop (Go to admin area and then edit shop)
When some1 wants to buy it (they will sent private message) you ask them for the TC coins and once they are sent, send them the IMG code in private message
If this is wrong, I opolagize, I am just now learning how so I might mess up...