Here is a work arround that will let you use any skin you download. The issue comes in with the bit depth of the images. The R4iDS skins need a 16bit image. All of the other skins I have seen are using a 24bit image. there is a free program called Paint.NET that will let you save the 24bit images as 16bit images. You will need to download the program and a plug-in for it. Here are the links to the software and plug-in. The plugin (a .dll file) needs to be coppied into the FileTypes directory in the install path of the application.
Paint.NET freeware software:
http://www.getpaint.net/download.html
Plug-in:
http://paintdotnet.forumer.com/download/file.php?id=5256
Just open the image then save as Bitmap (Low Color)*.bmp option. After it is saved, check the properties of the image file and you will see that it is now 16 bit. Do this for all of the images and the skin will work on the R4iDS card.
Hope this helps.