This function will tell you whether the object you are checking has a sprite or not, and if it does then it will return the index of that sprite, or -1 if it does not. Please note that this is not an instance function! You can have an object with no sprite while an instance of that same object can have one and vice-versa, or they can even have different sprites. You can set an individual instances sprite using the sprite_index instance variable.
|obj||The index of the object to check|
if sprite_index != object_get_sprite(object_index) sprite_index = object_get_sprite(object_index);
The above example will check the sprite index of the instance against the sprite of the object_index of the instance. If they are not the same, then it will assign the same sprite as that of the object index to the instance.