How to clean up a transperent background

Hi,
in JPEG file even in what appears to be a block of solid color (e.g. black) will have small random variations due to the "lossy compression" JPEG algorithm applies. You could still try the Image - Color - Substitute command, try to replace black with black, but mark the "Set opacity..." option there and set opacity to 0 to get transparent. Then use the "Color matching tolerance" slider at the bottom, move it right and press Apply to see if you can find optimal setting. It probably won't be perfect, you may need to clean up setting the transparency also on pixel by pixel level at a high magnification.
 
Greg