MAC INSTRUCTIONS: (please add to your first post)

To rename the files on a mac computer:

Control-click on the PT app (or right-click)
Open the resources folder
Open the images folder
Look for the three images named arrangementBG01, arrangementBG02, and arrangementBG03 - those are the standard background images.
Click on a file and hold down the mouse button for 10 seconds, then let go. Now you can rename the file.
Rename the image to something like arrangementBG01-backup
You may, instead, simply move the files to a backup folder of your choosing.

To prepare and use your own image as a snapshot background on a mac, follow these steps:
Open the image in preview, iPhoto, or any image editting program
Crop the image to 800x600 pixels
Save your image to the game's Images folder as a JPEG image (.jpg at the end. If it says that it already has another extension, click "use .jpg only") with the name arrangementBG01, arrangementBG02, or arrangementBG03
Open Plant Tycoon
Click on the camera icon
Click on Change Scene, if necessary, to display your picture

- Jazzo (And I used some of LadyCFII's original instructions)