Otherwise it may fall into a loop where you just sit at frame 0 all the time.
You might also want to set your image_index to the next frame (frame 1) when your alarm ends in addition to setting the image_speed. Image_index is which frame the objects sprite is currently at. Like if you wanted to switch your player from 'spr_idle' to 'spr_walking' or whatever. Sprite_index is for changing which sprite an object is using.
