This tutorial is intended for Photoshop. Mask brushes by
Sanami276@deviantart.com.
Images hosted at Image Shack.

Step oo1. Please download these brushes by Sanami276 if you
do not have mask brushes already; they are made for Photoshop 7.0+ and an imagepack is included.
These are what will be used in the tutorial.

Step oo2. Open a new 100×100 pixel document and load the brushes in Photoshop.
Next, pick a brush and stamp it onto the canvas. This is the brush I chose:

Step oo3. Next, in another document, open the image that you want inside of the mask.
I chose this image of the Japanese singer Gackt:

Step oo4. Copy the image you want to use onto your clipboard.

Step oo5. In the document that contains the mask brush, while holding
down the “ctrl” key, click the layer with the brush on it. The brush should now be
outlined with a dotted marquee. If not, repeat this step.

Step oo6. Now hit “Ctrl” + “Shift” plus the “V” key (ctrl+shift+v all at once) to paste the image you had on
your clipboard into the document with the brush to create the mask.
Your layers should now look like this:

Step oo7. Your final image will look something like this:

You can use this technique on any shape that you want to use to create a mask.

The End.