I saw this idea on Pinterest (my new obsession) and I tried to find the original link but it was a dead end. No fear, I figured this was really simple and dug right into it! Of course, I had to share!
First gather your materials. You'll need a transparent iPhone 4 case and the little insert card it comes with, a pencil, an exacto knife and cutting board, scissors and something you'd like for your design! You can use scrapbooking paper, your kids artwork, a print out from your computer, a magazine page, a label from a product... the possibilities are endless! I chose to make one from my blog header for this tutorial and for shameless advertising.

Size up your image. Be sure that the image you want to be seen is the right size for the case. Maybe you just want part of the image? Check your camera location. We are going to cut out that little hole. Is there some font there or part of the image that you don't want to lose? Better work on resizing or positioning. Next, take your card insert that came with the clear case and use it as a template. Position it over the part of the image you want to use.
Take your scissors and carefully cut out your image along the traced line.
Check out your camera location. We are going to cut this piece out.

Take your image out of the case and slip it under it. Make sure your non-image side is facing up to you (image against the table) and that it is going the right direction so that the hole you cut is where you want it. Line it up as best as you can and trace the hole for the camera.
Place your new case design inside the back plate of your transparent case, lining up the hole with the camera and place your phone in gently.