Then I opened the file again in Photoshop, changed its Mode to "Indexed Color," and selected "System (Windows)" for the palette (I could also have chosen "System (Mac OS)"). This changed the graphic to 8-bit (256 color) mode. I saved the new version of the graphic and when I used it as a custom POI icon, I finally got the magenta portions to display as transparent.Thank you everybody for your hints! But the above quoted was the one, that made the difference! It has to be an 8-bit *.bmp. When you save the file you have to choose this explicitly. Thanks again, now it works!