ZDoom: The Photoshop you code.
Actually, it wouldn't be a bad idea to have either of those, the effect looks kinda cool. Multiply would be a little more subtle than reverse subtract, though. You'd have to be careful using it, because ReverseSubtract would be nearly perpetually black against dark backgrounds - of which there are quite a lot in Doom, especially with dark light rooms, but it might work for certain special effects, though.
Multiply would work a whole lot better with darker images and darker backgrounds because you'll still be able to see both the image and the background if they are both dark.