For best image quality and speed of loading the page:

Images should be in JPEG, GIF or PNG 

We recommend Images that are 1920 x 1080 or smaller with a ratio of 16:9 

Before uploading the image to use on the home screen, run it through the online optimizer software here:

This online image optimizer uses a smart combination of the best optimization and lossy compression algorithms to shrink JPEG, GIF and PNG images to the minimum possible size while keeping the required level of quality.