Normally images are drawn in the order in which the instances are created. You can change this by setting the image depth. The default value is 0, unless you set it to a different value in the object properties. The higher the value the further the instance is away. (You can also use negative values.) Instances with higher depth will lie behind instances with a lower depth. Setting the depth will guarantee that the instances are drawn in the order you want (e.g. the plane in front of the cloud). Background instances should have a high (positive) depth, and foreground instances should have a low (negative) depth.
This will set the object's depth to -5.
depth = -y;
A code used when making isometric games. by adding this to all objects drawn, it will draw the closest objects (the highest y value) over the object further away.