This variable determines the speed in which GameMaker Studio 2 will cycle through the sub-images for the current instance sprite. The speed value given is a multiplier, with 1 being the default value, and setting it to 0.5 will half the animation speed, while setting it to 1.5 will double it. If the sprite used has no sub-images, this variable will have no effect.
with (instance_create_layer(x, y, "Effects", 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).