Can be done in a much easier way. Write script that will wait for a new file to appear in the pictures folder and as soon it does (photo was taken) will replace it with predefined proposal image. This should give you effect: take regular photo and the one that appears should be the predefined one.
If for some reason (I haven't tested this) it doesn't work as expected, instead of swapping the images, your script can simply call viewing application with the proposal image. Not as smooth as in the first case, but should do the trick.

[edit] inotify will be the way to go in this scenario, some inspiration: http://ubuntuforums.org/showthread.php?t=950939

If you are not a techi, let us know, I'm sure someone will help you with writing this.

[edit2] It would actually be really cool to include in the script taking the actual picture/series of pictures from the front camera right after the image is shown. Should show a nice set of I'm-starting-to-understand face expressions of the bride