I'm finding the best way to handle this these days is using billboards. Set up your scene lighting and camera angles, put in a subject you want, then delete everything but the character (and desk if you need it to interact with it). then render it without anything else in the picture. It should be really quick. You will use this render and an opacity map to texture a plane. You can put as many of those in a scene as needed without causing any overload problems for the computer - they are 2D. You can also rotate the camera around the subject and take multiple shots for future use building a library. If you take a look at some of the products RiverSoft is doing over at DAZ, that's exactly the technique he is using, and it makes doing crowd scenes a breeze. You can do scenes with 100's of characters this way, and they are all "high resolution" quality.

Riversoft's products are the "Now Crowd" series over at DAZ if you wanted to take a look at the technique. I make a lot of use of them as well as creating my own.

Edit: When doing the rendering, output to PNG - that will give you a transparent background but leave the shadows from the render so it will fit in with the lighting of the scene.