The best and most consistent results that I've been able to obtain with Wilbur so far require absolutely hard edges (a pure black and white map with no shades of gray). You can use any image as a selection, but you'd want to ensure that the image is pure black and white or you'll need to use Select>>Modify>>Binarize after every time that you load a mask. That extra step comes under the heading of "hard".

I like GIF because I have had problems with reading some PNG and BMP images as selections in the current version of Wilbur. JPEG files are lossy, meaning that the hard edges will be corrupted and that the binarize step will be required. Wilbur doesn't read TIFF files at all. That leaves GIF as the most commonly-available image format that Wilbur reads and that is lossless. It also supports 2-color palette options, meaning that black and white is easy to force. It also is compressed, allowing typical masks to be a few kilobytes in size. Plus, Photoshop's save for web option has a save-as-gif option that lets you directly specify black and white, so I can eliminate any need for the Binarize step during operations.

The technique described above is all done in Wilbur except for the preparation of the masks. The link I provided shows the step-by-step process of taking an image, what masks prepared from that image will look like, how to calculate intermediate sizes, creating the raw altitude layer cake, and a couple of iterations of the scale noise/fill/incise/precipiton/scale loop to get to the final size.