Does anyone know where I could find hi-res images such as images of forests, jungles, space/plantes/stars, etc., to be used as layer masks over some head shots?

I use ...

www.sxc.hu
http://www.morguefile.com/

.. when I need deserts or other things not available here, where I live. I do 99% composites so these sites have been good to me.
www.dreamstime.com is rather cheap with better overall quality of the images.

Also try the search. The same question was asked and answered 15 posts down.

Life's not free. :-)
But stock photos are pretty darn close to free.

www.istockphoto.com