Oddly enough sitepoint just posted a tutorial on how to do this, except their example uses photoshop. Perhaps photoscape will have something similar?
http://blogs.sitepoi...s-in-photoshop/
In the least, if it supports layers and some sort of layer blending modes, then setting the blend mode to a darken/multiple of some sort should do the trick. As a last resort, you can use the magic wand tool