Kurka, what you want (bitmap transparency) can be achieved by:

Under the transparency tab, turn "global transparency" to a value other than 0%. You'll probably want it at 100% transparent. This will make "variable transparency" available (it was greyed out before). After turning variable transparency on, go to edit function (right click on the sphere that appears). Under "kind of layer", change it to "mapped value". Click the button "Picture", and select your bitmap.

Hope that's what you wanted!