The sprite images for the RadButton are moved from the "Button" folder to the "Common" folder of the skins, but the Visual Style Builder searches the images in the Button folder.

The workaround is to extract the archive generated by the Visual Style Builder, move the sprites from the Common folder to the Button folder, archive the files again and import them in the Visual Style Builder. This allows you to edit them in the tool and after that you can download the skin and move the sprites back to the Common folder.