Also, it seems that I had the right idea about the colour pictures too - I did the same thing to red, green, blue, cyan, magenta, yellow, black and white: changed every one of those colours to transparent, one by one, and the whole colour picture became transparent with alpha channel zero, but pixels retained their original RGB value. So, the problem seems to be solved.
...correction to this. It does something to the image all right... but it's a bit weird and doesn't seem to work as I would think it should work.
If the photoshop has a way to edit alpha channel as a separate layer, by all means that sounds a far superior choice. There is a channel editor in GIMP too, but I can't even paint the alpha channel black for some reason. Edits to alpha channel seem to affect all other channels as well.
As for the medusa shine map - i suspected as much, thanks for confirming it.
