There is a way to mimic Masks in PSE. I have become addicted to collages and here's the technique I use.
- Put your original picture on its own layer.
- Create a new layer (CRTL+SHIFT+N) and drag the new layer below the layer of the picture you want to blend.
- Make sure you are editing the original picture layer and hit CRTL+G. This groups the two layers together. Don't worry that your picture disappears after doing this step! It will reappear after the next two steps.
- Select the outer edges of your original picture (Hold down the CRTL key while you click on the layer in the layer palette)
- Now make sure that you are editing the mask layer. Fill the selection with black. You should see your original picture now!
Now whatever area you erase on the mask layer will hide the same area of your original picture. If you erase too much, you can just paint back over the area and see more of your picture. If you want to play with blending modes in your collage, you need to change the blending mode of the mask layer instead of the picture layer.
HTH!