A sprite has to have at least one sub-image and this function will return just how many it has. Remember, a sprite sub-image starts being numbered from 0, so if this function returns 3 (the sprite being checked has 3 sub-images) they will be numbered 0, 1 and 2.
|index||The index of the sprite.|
image_index = floor(random(sprite_get_number(sprite_index)));
The above code will set the image index to a random frame based on the number of sub-images that the sprite has.