This read only variable can be used to get the number of sub-images in a sprite that has been assigned to an instance (if you need the number of sub-images for a sprite other than the one assigned to the instance you should use sprite_get_number). Please note that when there is (for example) 1 sub-image this variable will return "1" but the image_index of that sub-image is 0.
if image_speed > 0
if image_index > image_number - 1 instance_destroy();
The above code checks to see if the sprite is animating, and if it is it then checks to see if the current image_index is greater than the number of sub-images and if it is it destroys the instance.