Image speed

Explanation
The image_index variable defines the speed that the sub-image of an object's sprite progresses. Each sprite could have an number of images greater than 1, and the object 'animates' by moving a ceratin number of images per step.

Game Maker automatically increments the value image_index (and image_single in pre-6.x versions, but with conditions) by image_speed itself, then wraps the value around the maximum number of images, so if a sprite has 30 images, and the number 34 is reached, it is automatically turned into 4.

Conditions of incrementing image_single in pre-6.0 versions
The image_single variable is incrmented then wrapped around the maximum number of existing images in the sprite, however, unlike image_index, when image_single is equal to -1, it is not incremented at all.