If you don't want to change any of your existing skins, simply make a folder and put your "fruit-ryuuta.png" in it. Then make a file in notepad called "skin.ini" with the following information in it:
Name: Put a name for the skin here. (e.g. Kirby CTB)And put that in there too. Then you can just pick it off the skin menu.
Author: Put your name here.