This is a little utility I wrote to leverage the 32 bit icon unit I wrote. It loads an base bitmap image, and allows you to do some rudimentary alpha channel editing. You can also load the alpha channel from an 8-bit (256 color) bitmap. It will try to convert if it's not 8 bit, but you will likely get unexpected results.
Here are the images I used to generate what you see in the screen shot. The image with the black background is the alpha channel, and was created by simply inverting the colors of the original, then saving it as a 256 color bitmap. As you can tell, it's not the best method for creating an alpha layer, but it was quick, and worked fairly well.