A Mnemonic Image is a visual image that encodes data to make it easier to memorize.
An example would be using a visual image of a deck of cards to represent the number 52, because there are 52 cards in a deck. It is easier to hold a visual image of a deck of cards that it is to hold the number 52.
An example of compound images is the PAO System, where each 2-digit or 3-digit number has three associated images -- a person, an action, and an object. Elements of different numbers are then combined to make a compound image.