I would like to use an imported image as texture / shader. I can do that with the image map shader, but I want to apply the first to low altitudes and the second to high altitudes. Is it possible to (and how can I) limit the appearance of an image map shader to a chosen altitude or slope, and is it possible to mix 2 image map shaders?
Sure, just add the image map shader as child to a surface shader and set altitudes there. Or mask the image map shader itself by distribution shader. You can mix them with a merge shader, even add the distribution shader to control the 2 image map shaders (by height or whatever).
Yup, what Dune said...!