Recommended viewing zoom is 2x.
A little project I worked on for about two months, finding and pixeling one hundred varieties of trees. Making each one didn't take very long, probably about 20-40 minutes on each, but finding and picking them was a process that spanned two months. (And yes, some of these do stretch the definition of tree :p)
References and names for the trees can be found here: http://pastebin.com/BDdfFRBw