Making an actual floor with an actual image on it is one way. But I try to avoid that, because it isn't likely that the floor/wall edge is going to line up with your perspective.

Another approach is to use the ground plane as a shadow catcher. Poser's ground is set up for this quite easily by checking "Shadow Catch Only" in the material room shader for the ground. You can also make any prop do this. A prop can be helpful if you need to rotate the "floor", i.e. the background photo shows terrain that is not level, and you want to catch shadows at an angle that correspond with that.

Click here and here to see examples of where I've done this. WARNING: NUDITY!!! Both of those pictures are of the same scene, but from different camera positions. The ability to move around your figure while still automatically compositing your background photo with the right perspective is really helpful and easy with this technique.

However, the color of the shadow that your 3D stuff throws on the catcher will be unlikely to match other shadows in the background photo. This is tricky stuff, but I have developed my own "shadow catcher" shader that can reproduce shadow colors and intensities that perfectly mimic those in the photo.

See this thread for my custom shadow catcher. You can download the shader and use it on your own floor or on a one-sided square. Follow the instructions in the thread to see how to calibrate it so it matches your own photo's shadows.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)