Using plane textures for trees, people, etc.. in still rendering
Most of the time, pixel images are more suitable to represent people, cars, trees, etc... than 3D model. If these are not the main subject of your render, it often doesn't worth to model a precise 3D object. Moreover, it's very hard to make human or tree models realistics. In addition, they require a huge modelling which increase a lot rendering times and memory consumption.
So, there is a trick to use plane pixel images to fake 3D model in your render. In fact, we will use a cut out image and use it as texture on a plane facing the camera. The result will be the same as a postproduction in Gimp. The advantage to use this trick in Blender rather postprodction is less time consumption because wherever you move you camera, images will always face you, cast shadow on your 3D model and be cutted by a 3D foreground. However, if you use a tree picture shooted from an human height(~1.7m), you cannot use it to render a "from the sky view" of your model.
Prepare your texture
This is quite a hardwork here. You could find on the web libraries of these cut out pictures of trees and people to skip this part.
Otherwise, take a digicam and shot around you. Prefer to zoom max to avoid perspective effect. Especially for people, make sure feet are horizontal on pictures. Cut out the subject in your favorite image software (Gimp, Photoshop). The best technique is to use masks to have blurred transition between plains and holes. Once it's done save your picture in PNG for example to keep and alpha layer.
Tree cut out picture in PNG
Create your tree in Blender
To make it easier, this a Blender scene where your tree could grow in serenity. (A green grass plane with a sunlight and a blue sky :)
Render image Blend file of the start scene
First, create a vertical plane object in the center of your grass plot. Plane dimensions should be the same ratio as the texture size. For example if your tecture is 500x1000, your plane could be 2.5x5 blender unit.
Then Create a new material in the shading panel (F5). Create a new texture for this material, choose "bitmap" as type and load your tree picture. Tick "UseAlpha" and the "Alpha" button near the preview to see transparency. Is'nt that great?
Go back to the material panel. Tick Ztransp, Shadeless, put Alpha slider to 0. Tick Alpha in the "Map To" tab. That's it for the material.
Render image Blend file of the scene
Tips: If you don't see the tree transparent shadow on the ground, tick the TraShadow button of the ground material (not the tree material).
Make your tree face the camera
This a good start, but imagine you have a lot of trees surrounding you camera. How to make all of them face it?