If you have an old copy of Bryce lying around, the tools it has for editing terrain are ideal for creating instant alphas for Zbrush. Not only are they quick and easy to make, you get a realtime 3d update to give you a clue to depth etc. Good eh??
Bryce saves them out as .PGM files, which is a file used to represent terrain from satelite scans. However, Ivanview ( and photoshop with the right plug in ) loads these files up and converts them to .BMP or .TIFF. Pop them into the ZB Alpha Palette, they hey presto!