because large image sizes are so memory intensive, unless you have unlimited ram you will run into trouble with blender. it doesnt fail gracefully once it runs out, rather it will just die.
there is an add on script that comes to the rescue for rendering high resolution images, and it makes use of imagemagick.
not only be aware that high rez images take up ram while rendering, so do a high amount of vertex's in your terrain mesh, so use the multi rez modifier. i really want to be able to focus on this subject more and provide more options for optimising workflow.
shading of the terrain builds on the previous incline node setup so that it can render out detail maps automatically. also a type map can be exported aswell
the images are rendered to 16 bit tiff images.
thats all for now.. will clean up the fluff later.