Well...the "correct" way of creating normal maps for trees wouldn't involve using a photograph--it would involve using actual 3D geometry . If you're not too picky on what kind of trees you're using, you can always download a pre-made tree (i.e. http://trzyde.blogspot.co.uk/p/freebies_5.html), and render out a billboard with all of your maps you would need (Normal, Spec, Albedo, etc.). For better looking trees, and especially for hero trees (i.e. the oak at VIR), you should make actual 3D geometry (i.e. http://www.polycount.com/forum/showthread.php?t=126456 -- posts 12 and 23 offer some tutorial-ness ). I haven't yet created a tree that I'm happy with to put rF2 through its paces to see how much geometry you can get away with. So, I'd probably recommend using them sparingly at first, and certainly mostly only on high visibility levels only. Also, if you don't mind spending a few bucks, I'd recommend checking out something like nDo by Quixel or Bitmap2Material by Allegorithmic.
Ddo as well (legacy version is free) and I think some of the previous versions of the suite are becoming free to when they release their new version. http://dev.quixel.se/
