We take the example of flat images of butterflies, copying the individual butterflies and placing them in a new 16 bit, 100 dpi image.

To set up the 3D shape, we'll use gradients and darker tones for depth. Find the center of the canvas by turning on the rulers and dragging to snap to grid. Use the marquee to select half of the canvas and apply a black and white gradient. Put this gradient in it's own layer, and duplicate for the other side of your canvas after rotating.

To create a 3D bulge for the butterfly's body, select the boundaries of the butterfly, and in a new layer, use a brush to paint at 5% opacity in black. We merge this with the gradient background and blur the image a bit to get rid of the hard edges.

Apply 3d > New Mesh from Grayscale > Plain. This is a CS4 feature only. Use your 3D rotation tool and give your object a look. Drag the butterfly onto the original gradient layer, turn the gradient off, and save your file.

Now your butterfly has become a 3D object!