A question I am sometimes asked at shows is “Why does this image have the look of being 3-dimensional?” The image that receives this question most is Blue Bird, an image of a bluebird on fall arrowwood. The explanation is that there are two elements of this image that lend it a sense of being 3-dimensional. The first is the limited depth-of-field. That is, the foreground (including the bluebird) are perfectly in focus while the 
background (distant trees) are rendered only as impressionistic form and color. This situation implies distance and the mind interprets the scene as having something close and something far away. The other element contributing to the 3-d effect is the colors in the image. Warm colors (some of the leaves and stems of the plant) appear to the eye as advancing, while cool tones (the bluebird here) appear to recede. That combination of advancing and receding color right next to each other are interpreted as subjects being close and more distant.
There are other ways that a two-dimensional image can have a three-dimensional look. But of the ones that I place in my booth, this particular piece receives the most remarks.
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The red-orange against the bright blue is a lovely contrast in that photo. I sometimes paint with a limited depth of field for the same reason. I figure if it works with photos, it should work for paintings, too.