There are dozens of image formats available, but Google search supports only these formats: JPEG, PNG, WebP ... as dictated by your CSS rules, rather than assuming it takes up the whole viewport ...

Mobile-oriented, fast and extensible jQuery plugin for lazy loading of images/videos. Currently tested in IE 6-11, Chrome 1-47, Firefox 1.5-43.0, Safari 3-9, Opera 10.6-34.0, iOS 5-9, Android 2.3-5.1, ...