Posting images with a Google Photo's URL is fairly easy but there are a couple of steps for it to work correctly.
The common trap for young players is to make sure the web address (URL) is a link direct to the image and not to the Google Photos page containing the image.
This is the usual error that results in a failed image. Google wants to promote itself to your friends so the default 'Share' link that they provide is to a Google page containing the image and not a direct link to the image. This default page link will not embed the image on any forum nor any web page.