Well I know Photobie will allow you to combine multiple photos in the form of layers and then merge them into one -- but I have not really used the program and cannot speak for its ease of use (or lack thereof).
There's a tutorial, "Club," and it's freeware software (for personal use).... so it may be close to what you're looking for.
Photobie: free image editor, photo editor, screen capture ( screencapture ), GIF animation, flash jukebox, digital scrapbooking, photo viewer, icon editor etc