Thanks Master, but this is what I already did, I am sorry I was unclear in explaining. What instead I want to do now is to use the heightmap to project shadow within photoshop, in this way I can tinker with bevel and emboss and drop shadow (and avoid or lessening the problem of the fractal pattern at closer resolutions), which I can't if I use the previous method.
Isn't there a way to transform a level, or its content, into a level mask?