Image index

Explanation
The image_index local variable is the variable that defines the current subimage of the current selected sprite. A sprite in meaning is an graphic with multiple sub-images. Every sub-image in Game Maker has an index, starting from 0 (which is the first image) and ending with whatever the user decides the number of sub-images should be.

Since Game Maker 6.x, image_index alone controls the position of the image in the sprite of the object. In previous version, another variable: image_single existed.

A sprite of an object, by default, is constantly moving, and therefore image_index keeps changing, and therefore it cannot be used to pause a sprite (unless used in step), what can be used, however, is the Game Maker variable: image_speed.