Outlining Brushes

Begin with any image. Create a new raster layer and place your brushes.

Next, select the raster layer and press Ctrl+C to copy it. Open a new, bigger, image. The background color of the new image should be the color you want to outline your brush.

Paste the brushes as a new selection on the new image. Then, go to Selections > Modify > Expand and set the options to your liking. When you're finished, press OK, then press Ctrl + C to copy your outlined brush(es).

Open the first image again, paste the outlined brushes as a new selection, and press Ctrl+D to de-select your brush.

In case you were wondering, the fonts I used were "Plastique" and "Pea Jenny Script".