I just tested it to see how to do it. I'm more familiar with flash, but it's about 100x easier in maya.

In flash/or after effects you have to simulate the 3d movement by squashing the cowboy and then playing with his scale to have the illusion of the cowboy moving away from camera - the ground is a masked vertical pan, if you want I can send you the file, but i'd recommend to do this in maya.

All you need, is creating a plane and then in its attribute editor go to lambert1, click on checker-mark beside color, click file, select path of your picture (I used png picture cause you want the transparency around the character).. that's all. Then just animate the plane pop up, do and overshoot for a stronger effect (I had him go to 95% and settle to 90%, but it's up to you, also adjust the timing in the graph editor, I had it on default but it should be like this: sharp ease out, ease-in-ease-out)

As to the ground, i'd just make a big sphere, if you want a nice rounded horizon.

See the link for the test: first part is done in flash, second in maya - rendered through perspective view...

cowboy.mov - YouTube