Create a new layer above the artwork that you would like to add sparkles to. You can create this new layer above one, or many layers below it. To create a new layer, go into the Photoshop layer menu and click the icon that looks like an arrow in the upper right of the box. One of the options in the dropdown should be to create a new layer. Choose this, and name the layer something that you think will help you to remember its the sparkle layer.

Select the paintbrush tool from your toolbar. Once your brush is selected you should go to the brush menu (usually located in the upper left of your screen) and choose a wide, fluffy looking airbrush. You can choose a very large airbrush for particularly brilliant sparkles, and smaller airbrushes for sparkles that are more diminished.

You are going to want your brush choice to be solid towards the middle, and fluffy towards the edges. Where you would like sparkles, go ahead and tap your airbrush once or twice on the canvas. Since my example image was somewhat light, I have duplicated the results on an easier to see background.

Sample a color that's a little bit lighter then the one you were just working with, and go with a fluffy brush roughly 1/2 the size of the brush used for your halo. Tap directly in the center of each of your larger circles, showing that the light is intensifying as it moves inward.

Finally, pick out a very small soft edged brush and select a color that's very near to white. Tap once or twice directly in the center. In the dragon image to the right I also tapped a few spatters of light outside of the halos, ones that aren't quite as intense and are just sparks.

For particularly bright sparkles, you may want to go the extra mile and create another new layer. Use your line tool and draw a star on top, then use your smudge tool and smudge out the edges. A wee bit detailed for tiny stuff, but for a particularly brilliant magic effect, it works rather nicely.