Attila Soos
Omono
For typical silhouettes of trees, take a look at Petrides' A Field Guide to Trees and Shrubs which has a section devoted to that. They are pictures of trees as they would (or might) look growing by themselves in the open. Keep in mind that trees "in nature" will be a LOT messier, with odd and broken branches, etc.
Here is an example:
Yes, I always select the tree guides with the pictograms of the trees that you are showing as examples. It gives us a good idea of the natural growth habits.
The disclaimer is, of course, that you take a tree from its ideal enviroment, and place it into a harsh habitat, like a mountain cliff at higher elevation, and the growth habit may change. So, there is always room for getting creative with the design.