[SergeNL]

Found a workaround..
Just use the default ugly Pink color for transperancy.
Other colors are supposed to work, if they are the same as the color of the top-left pixel. But this does not always work.

How to save the pink color in Photoshop?
Load Main.bmp of Base Skin or Sample Skin in Photoshop.
Click Foreground Color. Pick the pink color, and click "Add to Swatches". Close Foreground window.
Next time you'l find the pink in the Swatches window (next to Navigator)
In Gimp this may work too.

How to create an 'Any Shape' Main.bmp?
Load a picture in Photoshop.
Select what you don't want to be visible. (keep Shift Key down to add selections, or Alt to deselect)
Click Foreground Color, and pick the ugly Pink from the Swatches Window.
Menu Bar > Edit > Fill.
Menu Bar > File > Save.