I can't for the life of me remember where I saw this tutorial, but you can try the following:
1. Load terrain
2. On rendering control - switch to terrain units, set camera x and y to half terrain size (i.e. 256 for 513x513), uncheck fixed height above surface and set z to 8192.
3. Set camera orientation to 0, -90, 0. Click on camera settings and set zoom to 32.
4. Adjust sun position to taste - high noon for no shadows...
5. Set image size to something square (512x512, whatever)
6. Render
7. Save
I think that's everything, let me know what I forgot...
Ta,
Anthony