image_speed
The speed with which we cycle through the sub-images. A value of 1 indicates that each step we get the next image. Smaller values will switch sub-images slower, drawing each sub-image multiple times. Larger values will skip sub-images to make the motion faster. Sometimes you want a particular sub-image to be visible and don't want the program to cycle through all of them. This can be achieved by setting the speed to 0 and choosing the correct subimage.
GameMaker 6.1 Manual.