Each picture has two images (screens) - the visual screen and the priority screen. The visual screen is what the player sees when playing the game. Views are placed on top of the pictures during the game and can be moved around the screen. The priority screen is what makes the game appear to be 3-dimensional. On the AGI screen, there are 12 priority bands in which objects can be placed. The priority given to different parts of the picture (i.e. the colour of the pixels on the priority screen) determines where in 3-dimensional space objects appear. For example, a tree placed right at the front of the screen would be given a priority of 15 (the highest priority). A wall which is far away in the background would be given a priority of 4 (the lowest priority). If an object with a priority of 12 is displayed on the screen, it would be behind the tree but in front of the wall. The priority of a view is usually determined by its y-position.
Colours 0, 1, 2 and 3 on the priority screen are used for "control lines" which determine where the ego (the object controlled by the player) can walk.
Partial AGI picture tutorials can be found at the following sites (note that these links may become unavailable in the future):
http://weremoose.tripod.com/Tutorials.html