Terrasame i dont think that the File Types or Polycount are the problem... He uses 256*256 Bitmaps (these must be under 1 MB per file) and on the Screenshot above you can see the Models are low poly...
Maybe a File is broken? So open and resave all files (Bitmaps) and reload them in WED or MED...
cYa Sebastian
Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE
