Argh, I am with Phil, I also thought you meant 2D people in a 3D environment, but on re-reading it, realized that you wanted 2D people on a 2D background. Photoshop. There is not enough information for automated work here.
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Here is my previous post:
Well... depending on what you are hoping for, you could use a trick to get in the right direction.
Fundamentally, the others are right: if you have just 2D data, there is no easy way to add 3D-dependent information to it or the scene.
However, where there is a will, there is a way. First of all, the people need to be carrying their own shadows (on themselves), or you will need add it in Photoshop.
The shadows of the people on the environment, however, can be faked. It would be possible to model a really crude person, make that person invisible in the scene, but able to cast shadows. That way when it renders, the shadow will come from the proxy, and the look of the person from the photo/cutout. The feet would need to match closely, but we are surprisingly bad at telling exactly what shadows (or reflecions, or ...) should look like. Basically anything beyond directly visible objects can be fudged quite a lot without us being able to tell.
As far as shadows from the environment onto the person goes, that could be a little trickier. Since the visual appearance is given by a 2D cutout, any shadows falling on it will be flat. You might have to resort to Photoshop here again too, to fix up the details.
Architectural visualisation uses this type of workflow a fair amount, to my knowledge. Aren't there helpful CG sites for this audience? Failing that, more information could be found in the forums on sites like cgsociety.
