Image single

Explanation
The image_single local variable is the variable that defines (in a different way than image_index) 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 main, first, image) and ending with whatever the user decides should the number of subimages be.

A sprite of an object, by default is constantly moving, so whatever was the value set to if it was a positive integer less than the number of sub images, the sprite would keep animating. However, without the need for image_speed, setting image_index to -1 pauses the progression in the sprite's subimages while keeping the actual subimage of the object's sprite.

Remarks
Since Game Maker 6.0, image_single is no longer documented in the help file and is deprecated, though can still be used in the program. The new variable for this is image_index.