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.