This variable determines the speed in which GameMaker:
Studio will cycle through the sub-images for the current
instance sprite. A value of 0 will cause no cycling, while a value
of 1 will cycle one image per step, but you are not limited to this
range as you can use values greater than 1 to animate faster
(frames will be skipped), decimal values to animate slower (frames
will be shown over various steps) or even negative values to
reverse the speed of the animation.
with (instance_create(x, y, obj_Explosion))
image_speed = 0.5;
The above code creates an instance of the object "obj_Explosion" and sets its image_speed to 0.5 (effectively halving the running speed).