I'm not sure, but this way seems to come out great. It may take a little longer, but it's all worth it.

1. Open any two images, and post them on a background. Arrange them anyway you want.

2. Goto the layers pallete. At the bottom, there should be a square with a circle inside. Click it for both of the layers.

3. Instead of the paintbrush next to the thumbnail of your images, you should see the square with a circle inside. If you don't click on the white box next to the thumbnail of your images until you do see it.

4. Next, click your gradient tool. On the two color boxes under all the tools, make sure black is the first color. Next, make sure that the gradient color on top under the drop-down menus is set to black to transparent.

5. With your gradient tool, click away from on of your images, than drag the line of the gradient effect into the picture a little bit. You may have to do this a couple of times, but you should notice that one of the edges will be gone. Do this to all of the images. Once you're dont, move the images together.. and you're done.