One other method of giving your area of interest that luminous effect is by keeping the edges of the painting either in complete shadow, or just generally darkening the areas that are closest to the edge of the painting, even if they are not in shadow.
Photographers call this effect an “edge vignette”. To me this is one of the most subtle yet effective techniques I have ever seen in improving the impact of an image. I apply this effect to almost every digital photo I process, and paint it into many of the paintings I execute. Here are a couple examples:
A photograph without and edge vignette:
Nice image, but a bit flat.
The same image after applying an edge vignette:
Viewers eyes stay away from the edges.
Here is the same type of thing but to a lesser degree, in oil paint:
“Point of Impact”, 11x14 inches, oil
The thing to remember is, when going for this look in oil paint you kind of have to plan ahead and place your darks first, working up to the lights. In processing a digital photo however, this is one of the last and easiest steps to take.