The idea is to not just squeeze or crop the image as it is resized to fit (say) a browser window, but to remove less informative parts so that important objects and people remain. the browser compresses the image and the designer has made Y a preferred seam. We will tend to see X and Z, and likely never think of Y. This is a form of manipulation more subtle than just removing Y, since the designer can now truthfully say that the photo is authentic