At that res, each pixel is 7.8m across so really its more of a tree like texture than actual tree shapes. You might be better off getting hold of some aerial trees at a higher res than that and resampling them down such that they are approx 7.8m per pixel and then using that as a texture fill into the areas you want to put trees down. Since NASA images are public domain I am sure you could get something from them for terrain and cities at about that scale.
The NZ land info service has a lot of high res photos like that too. They are not public domain but do allow some reproduction usage with acknowledgements. So have a look at these for 25m or 2.5m trees.