Multiple images can be provided for the same scene, and the game will display one at random.
The mechanism for providing multiple images for the same scene works by iterating over a sequence, so to create a sequence for a scene, you would do something similar to the following: monster-ceraph.png would be renamed monster-ceraph_1.png. The additional files would then be named: monster-ceraph_2.png, monster-ceraph_3.png ... monster-ceraph_99.png, etc. (basically, add _{num} before the extension.)
The images must also be sequential and start at #1, the system only looks for img 3 if img 2 exists, so if you have gaps, it will stop autodiscovery.
Lastly, even if you have monster-ceraph_1.png, monster-ceraph.png will still be loaded, so if you just copy the image, you may get multiple instances of the same item.
Images in a set for a scene can have different extensions, provided the actual file-name properly matches the sequence topology as described above.