I'll explain a simple way to do what you want.
Find an image that you want as the background through Google images. Try to find a size that will suit you.
Find a simpler winamp skin that has a basic layout that will fit into the image you selected from google images.
Now all you have to do is rename your image found on google images to match the name of the background image used for the winamp skin you picked.
Swap the google image for the background image in the winamp file.
Done! simple but it's really your best shot at getting one made.
