try a higher resolution for better detail. maybe 1024-full windows/rendering editors render globals. but make sure its ticked as render globals and not perspective. to do this go to render in the pull down menu, where modeling animation etc and situated top left. then go to the render tab/test resolution/global.
to compose i would of thought render your tree and save as png which will save it with no background tranparent then open it in adobe, and create a new layer and add the tree to the new layer from your original house render.