get random picture from a google drive folder
create a new script in a Google site "App Script" name it getSitePictureRandom copy the code.gs and index.html to the new script save it Deploy as web app copy the url generated
open or create a new page in your google site edit the page add a gadget by URL, using the saved url from above
store images to be randomly selected in your google drive, the code uses the base folder /web-resources/images from there sub-folders "locals","intl","afar" can be selected see the code defining this...
var imagelibbase = "/web-resources/images";
var imagelibs = ["locals","intl","afar"];
.... so drop some images in these folders. If you give the images a description, that will appear as capture text under the image