Image Background
The widget is named ThemeKit Image Background since it is an image set in the background rather than a background image. In general a background image should not be used in responsive page design since the image file required is way to big and will seriously compromise the page download time on mobile devices.
HTML5 Picture
The image is inserted in three sizes for the various devices using the <picture> element which has the added advantage of allowing an alt text attribute for keywords for the search engines and screen readers.
Rather than be stretched to fill the whole browser, the image has its width and height set in viewport units.
Images
Before importing into EverWeb the images were sized to 1200 x 800px 800 x 800px and a 9:16 aspect ratio image at 375px wide for mobile phones in portrait mode.
Overlay
The optional overlay has controls for the background color and its opacity and has a fade in animation with controls for the animation time and delay.