You can use an image as bump map as well as displacement map. It depends on what you want to do. Displacements change the geometry whereas bump maps don't.
Let's say you have a black image with some white spots. This will create some elevated regions on the figure. If you connect this image to the bump channel then it only looks elevated whereas the outline (and the shadow) doesn't change.
If you connet it into the displacement channel, then the outline is being canged.
Note, that the displacement is quite a strong parameter. If you want to use a displacement map on realistic human figures then you should adjust the strength to 0.001 or so.
If you turn the value too high then the geometry (or at least the rendered image) tears and you see those artefacts.

Within Poser 4, there was a special file format *.bum. I don't know wether this can still be used with P6 but is was nothing other than an ordinary image, *.bmp as far as I remember. There was no big secret behind this.

Hope this helps a little bit.