my guess is that you're using the horrible example under the help files for
set image colorkey:
set image colorkey 255,0,0
load image "car.bmp",1
cls
paste image 1,100,100
do
loop
end
which is junk...
instead, try:
set image colorkey 0,0,0
load image "test.bmp",1,1
cls rgb(255,0,0)
paste image 1,0,0,1
do
loop
which definitely works with the .bmp that you provided (i renamed it to test.bmp) where i first clear the screen with red to show that the (true) black is transparent/allowing the red to show through.
(note the final
,1 transparency flags set when loading/pasting the image)
there are points in your "black" that aren't true rgb(0,0,0) so you need to reconsider how you're building/converting your image, as well:
