Alright, I should have expressed myself a little more clearly: I solved the problem by switching on the clamping and rendering the quads "overlapped".

My textures were 512*512, so they match the criterion of 2^n*2^m. Textures whose measurements are not powers of two are not displayed anyhow, just like I didn't bind them.

And they are 512*512, generated with Terragen, TGA format. The real problem is the size, even compressed they have a size of ~1.2 MB together.

EDIT: I'd suggest NOT to render the landscape as I did. These mountains look good, but they are a little blurry and they disturb the fogging-effect.