What is a good technique to make a realistic gray backdrop in Photoshop?
Lets say I have a cut out subject on its own layer, photographed with 2 umbrellas, 1 stronger from the left, and a softer fill light from the right. I would like to place this subject on a gray backdrop as if it was shot in studio. It has to look realistic tough, so not a matte gray background, nor a simple gradient. It has to contain shadows, noise, light fall-off and all it needs to make it look as realistic as possible.
As an example (not mine): how to get from this: http://www.officialpsds.com/images/thumbs/ROSE-DRESS-psd73460.png to this: http://onlyfreewallpaper.com/view/rose-dress-girl-1920x1440.html
lets pretend the background in the second image is not the orignal one but created in Photoshop.