1) We will be using this poloroid from http://my-givenchy.livejournal.com/
2) Crop your base to 100x100
3) Go to Image>Image Size
4) Paste your base as a new layer
5) Set your image to lighten (you can move it to wherever you want)
6) Use the Marquee tool around your image to make sure it doesn't show on the border of the poloroid
Now you can decorate it however you want! Have fun!
Here's my final product: