The read-only y property of the HTMLImageElement interface indicates the y-coordinate of the <img> element's top border edge relative to the root element's origin.
An integer value indicating the distance in pixels from the top edge of the element's nearest root element to the top edge of the <img> element's border box. The nearest root element is the outermost <html> element that contains the image. If the image is in an <iframe>, its y is relative to that frame.
In the diagram below, the top border edge is the top edge of the blue padding area. So the value returned by y would be the distance from that point to the top edge of the content area.
For examples, see the x property page.