G’day!
I have a small suggestion for a good source of where to find alot more base material to make good looking useful alphamaps.
Make use of all the highres satellite photographs posted daily on the web, pictures like this one example i posted below, you could extract parts of it and use as a alphamap for skin or something else 
the possibilities are endless.
To find more just make use of Google, i’ll link some sites here to highres pics later on if anyone’s interested.
Links
http://earthobservatory.nasa.gov/Newsroom/NewImages/images_index.php3
Cheers!
/ Max
hehe i hope the idea is useful for ya