Adding images is quite simple.  The easiest way is to use clipping masks.  Simply follow these steps:

1. Open the template (.psd file) you would like to insert your images into with Photoshop.

2. Select the layer you would like to place your image into in your LAYERS palette, it will be called “IMAGE LAYER”.

3. Open the image you would like to place here into Photoshop. FILE ->OPEN from the top menu in Photoshop.

4. Select the MOVE tool (Keyboard shortcut “V”) from your TOOLS palette.

5. Drag the image onto the collage template. It should now appear directly above “IMAGE LAYER 1″ in your LAYERS palette.

6. Now, choose LAYER -> MAKE CLIPPING MASK from the top menu in Photoshop. This will insure that your image will only appear in the frame which you intended. You can also select the “create clipping mask” option from the top right arrow of your layers palette, shown here:

7. If your image does not completely fill the frame, or is too large in the frame, you will need to resize it. To do this, choose EDIT->FREE TRANSFORM->SCALE from the top menu in Photoshop (Keyboard shortcut “Control+T” or “Apple+T” on a Mac).

8. Hold down the Shift key while dragging out (or in) the handles that appear on the edges of your photograph.

9. When you are happy with your photo’s position, press “Enter”.

Repeat this step for any subsequent image layers.

INSERTING IMAGES USING THE “PASTE INTO” METHOD (if you’re unable to use clipping masks in your version of Photoshop)

1. Select the image layer you would like to populate in your LAYERS palette (It will be called “IMAGE LAYER 1″, “IMAGE LAYER 2″, etc.).
2. COMMAND+click on the layer mask attached to that layer to select it.  You should now see marching ants around that image layer on the template to show that it’s selected.
3. Open your image and choose SELECT->ALL from the top menu (or COMMAND+A).  You will now see marching ants around the whole image to show that it’s selected.
4. Choose EDIT->COPY (or COMMAND+C).
5. Go back to your template, and choose EDIT->PASTE INTO (or COMMAND+SHIFT+V).