More a tip for newbies:
Another way to add a soft natural gradiated background to images is the studio photographers trick of a curved background plane receding from the viewer. This is often called a "cyclorama".
The ring 3d tool can be used - set to 180 degrees - then stretch it's width so it's like a curved canvas. You may want to experiment with other curve parameters for different effects.
In this very quick (and dog rough) example above everything is on one layer.
However setting the cyclorama to a back layer and baking it without the other layers showing will stop it from recieving shadows from layers and objects infront. Graze spotlights could also be used and baked as part of the cyclorama background. These throw highlights in the background against the main shadows creating by the fill light and will therefore accentuate the forms of the objects.
Hope this may be of use and interest to some folk.