The Game Maker help file is full of a list of all the commands BOE. If there's a command you don't understand, all you ahve to do is type it into the help file's search bar and it will take you to the definition, which will also define the way to set up the command. So if you were to type in:
image_speed
It would take you to the definition, which would be:
image_speed defines the speed of animation in relation to the room speed.
image_speed(digit)
Also, side note: image_speed * room_speed = speed of animation.
If you need anything else, hit me up any time.