This read only variable returns the height of the sprite that has been assigned to the instance. This height is returned in pixels and will be dependant on the image_yscale. If you need the un-scaled height you should use sprite_get_height.
For functions relating to sprites, see here.
if sprite_height != sprite_get_height(sprite_index)
image_yscale = 1;
The above code checks the height of the sprite assigned to the instance running the code against the height of the sprite resource and if it is not the same, it resets the image_yscale to 1,