Your most of the way there, your problem lies with transparency
Some areas (small dustings) of the shakuras_sand texture are 100% transparent When you go 'Remove Alpha Channel' in gimp you effectively convert all alpha to white (the same as sliding a solid white background under the image) This washes out the color difference and worse, the 100% alpha areas become 100% white, killing all color that was there on the diffuse map (Those strange teal specs on the ground) :(
- What you want to do is right click the layer, go 'Add Layer Mask...'
- Pick 'Transfer Layer's Alpha Channel'
- This splits the layer into 2 sides, Color and Alpha..
- Now right click and go 'Delete Layer Mask' to remove the Alpha without compromising the color
Alternativly you can just copy the origonal 1024x1024 pixls, create a new 2048x2048 image, paste the pixls into the new image 4 times and you should be able to move and play with them even though you have alpha in there :)
I'm not sure what alpha is used for in terrain diffuse.. possibly something to do with shaders or lighting, but without adding it back in you will still have normal looking ground.