Very simpel; import texture in image map shader, set displacement, set size you want it to cover, set projection if other than default Y, and attach to either compute terrain for major displacement, or as child of surface layer before compute terrain, or if you want the colors/greys of the image map to drive displacement attach to displacement input. If you only need the colors, forget about displacent and set as child to surface layer after computer terrain and base colors.
If needed as a repeated texture across large terrain, set repeat X and Z.