Bryce seems to know what to do with portable network graphic files (.PNG). Resaving the image in that format is probably the fastest fix.

If you use the image in the second window and bryce doesn't recognize the alpha channel you could get a ghost image instead of solid. That's because Bryce is using luminance instead of the alpha to determine transparency. I used to make a color and a seperate black and white mask for everything. Now I stick with PNGs because it's just one image to track.