You probably needed an alpha mask, to put on a plane (for a tree or something). Because alpha channels are made of grayscale values (256 of 'em), you probably converted your jpg to grayscale (via modus -> greyscale) and saved it in an appropriate file format. You just reminded me I still do have to check the oil in my car. I am glad they did put yellow paint on the handle of the long thingy you have to pull out of that large chunk of iron in the front of the car that gets hot after driving. (I guess I am one of those people)