Now available in the new Google Sites
Cover Pages - new Google Sites
Make cover pages in new Google Sites to get a full-screen image when the page loads with content beneath
Make a cover page
In the page header use the Header type button
The header area will increase in size to fill the editor content area
Open the preview and you will see at the bottom of the header is a downward-pointing arrow to jump down to the content.
You need to upload an image with high enough quality to be displayed full-screen on any screen resolution, e.g. this sites gets visitors with 2560x1440 screen resolutions and higher.
Since the images will be displayed full screen on all devices a squarer aspect ratio is better than rectangular.
The background image should not contain any important information as it may not be visible on all devices (e.g. compare the desktop view with the mobile view in the preview.
The down-arrow does not appear if you have only a header and footer with not content. Put a text box in the content area, even if empty, and the arrow appears.