Judging from your Discord post, you already found a way to do it, that’s great!
In any case, you can use the Image Mapper to assign your material to the rocks. Best way to do it is to create a Transfer Map first using the Mesh Bale node. How you could try to approach this:
Try scattering on a plane, then bake a plane above it to capture the Transfer Map which you can then use with the Image Mapper.