Make a back-up of all of their images and put this aside. Rename it something like Copy of images. Make another copy of their image folder and replace the images with pictures that have nothing to do with their project. We always find that nude photos or copies of Dilbert cartoons. Not to worry, Scott Adams will not mind. Much.
Carefully rename the prank images with the same file names if the images you are replacing. Copy the files into the original directory.
For heavens sake, be sure to make sure you have a backup before you start!