The ElementTiming API will allow developers to know when certain important image elements are first displayed on the screen. It will also enable analytics providers to measure the display time of images that take up a large fraction of the viewport when they first show up.
Docs: https://docs.google.com/document/d/1blFeMVdqxB0V3BAJh60ptOBFY7cJSXnf7VyW3wspbZ8/edit