I haven't really checked it on WB11, but I think this still applies. 2 options:
1- Don't define an image, and it will use the System Colour ButtonDkShadow with a little bit of transparency.
2- Define an image in Window frames / UIS2 / Non captioned window edge. I had better luck using a bmp or a png/tga without any general transparency (maybe some on the corners), transparency seemed to cause some graphic remnants while mousing over. The default image count is 4, but I used to use 2 with a left and a right (the "help" tab and the note at the bottom of the image tab indicates 2 states).