A few days ago Frank Llosa, Virginia Real Estate Expert, wrote about the insanely great idea of creating photo collages for your listings: Excellence Comes Standard: Main Photo Collages & other Photoshop adjustments
In case anyone would like some basic step-by-step instructions on how to do this, here you go: (I used Photoshop Elements 5 for this example)
1. Open the image you have chosen for your main image. Since mine will be used on the web only, it does not need to be a large file, so I will resize it.
2. Resize to 640 pixels width x 425 pixels height.
3. Leave your main image open.
4. Open your second image. Resize to 175 pixels width x 131 pixels height. Apply a drop shadow (Adding A Drop Shadow In Photoshop Elements 5) (Adding A Drop Shadow In Photoshop Full Version) or border (Photoshop: Adding A Border ) if desired.
Hit Minimize.
In Elements, this will send the image to the "holding tray".
5. Open your next image. Resize. Apply a drop shadow or border if desired.
6. Open final image. Resize. Apply a drop shadow or border if desired.
7. Click back in your main image. Hit Layer -> Duplicate Layer.
8. Click the move tool. 
9. Click on the first image for the collage, and drag it into the image image. It may jump towards the center of the main image. Just use the move tool to position it where you want it.

10. Click your second collage image. Drag into the main image.
11. Click your third collage image. Drag into the main image.
12. Make any adjustments needed in positioning. More on using the move tool here: Photoshop Elements: Multiple Images and here: Creating A Simple Blog Banner In Photoshop
13. Save the final image with a new file name.