UNDER CONSTRUCTION!
PHOTOSHOP
1. Open both the mask and the photo/background paper you want to apply the mask to in photoshop.
2. Drag the mask onto your photo, so the photo is the layer above the mask.
3. Now move your cursor over to the layer menu and while holding down "alt" on your keyboard, hover between the two layers.
You will see the cursor change to a little arrow with a "basket" or two overlapping circles.
Click when that appears.
You should now have your mask applied to the photo. You can now drag your photo around on your canvas to adjust it to your liking, but the mask stays applied.
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OLDER PHOTOSHOP VERSIONS
1. Open a photo and the mask you want to apply to it.
2. If necessary, rotate the mask so it is the same orientation as the photo. (Image > Rotate Canvas)
3. Activate the mask image, go to Image > Image Size. While the Image Size dialog is on-screen, go to the Window menu and choose the file name of the photo you'll be applying the mask to.
4. Click OK and the mask will take on the same dimensions as the photo.
5. With the mask active, Select All (Ctrl-A/Command-A) and Copy (Ctrl-C/Command-C).
6. Activate the photo and convert the background to a layer by double clicking the background in the layers palette.
7. Activate QuickMask Mode by pressing shift-Q.
8. Paste the mask. (ctrl-v/Command-v)
9. Exit Quick Mask Mode by pressing shift-Q again. You will now see the marching ants indicating a selection.
10. To reveal all white and gray areas of the mask, click the layer mask icon at the bottom of your layer palette.
11. To reveal all black areas simply hit the delete button.
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PAINTSHOP PRO
1. Open the mask and the photo you wish to apply the mask to.
2. For the photo you'll be applying the mask to, right click the Background in the Layer Palette and choose Promote to Layer.
3. Choose Masks > New > From Image for versions 7-10. For versions 11 and above click the little arrow next to the "new layer" icon in your layer menu and choose "New Mask Layer" and choose "from Image".
4. In the source window drop-down menu choose the name of the mask image. Create mask from source luminance should also be selected. Depending if you want the black or the white/gray areas revealed, click or unclick invert.
5. Click OK and the mask is instantly applied to the layer.
If you are looking for a JPG mask tutorial for PSE, please see Melinda Staley's tutorial here.