here's a quick shot:

1. add your image
2. add your next image
3. click the quick mask icon
4. fill the mask with BLACK and paint away with WHITE the area you want exposed (or what you want to be seen...)

REMEMBER:
Black=masked
White=transparent

Using the Quick Mask mode lets you edit your selected area on the layer as a mask without using the Channels palette and while viewing your image. The advantage of doing it this way is that you can use almost any Photoshop tool or filter to modify the mask.

You could also add a new channel and add the mask you created into it. it's just the quick mask is faster...

:)
retrocity