breme330xi, it's not that easy removing the mountains of that image still let it still look realistic.

but: have a look on this site:
http://www.cadtutor.net/download/raster/sky_01.php
there are 4 pages with different sky images.
or try http://images.google.com/ looking for a sky picture that you like.

Then, send me the link to the .jpg and I will create your personal horizon image for the BMW satnav for you.