WebGizmos would be right Doortech. Best way to do that would be to pick a skin, note the authors name and, depending on how you use the graphics and if said author is still around, older skins for XP that have been around awhile whose authors have moved on cannot be used except for personal use, you need that authors permission. If for personal use only you can mod any skin to your hearts content. For any other use there are copywrite issues and they need to be respected. So....that said......
TrickTrackersMJN - the type of images that support transparencies are .png and .tga to name two. You can use Google images to find one you like or you can use a template WB, the Diamond skin is an excellent choice, open one of the images in your graphics program. Keep the overall dimensions but change the color, borders and other aspects of the image. You aren't violating copywrite if the only thing that remains the same is the size of the image. If you're using Photoshop there are two ways to adjust the transparency, the fill mode and the opacity mode. The opacity mode adjusts the transparency of the image as a whole, the fill mode changes the interior of the image, i.e. inside the borders. Long story short the images you create are your own and can be uploaded. Tell us what you need and we'll do our best to help you.
BTW - did I say that right? 
