Because the correct address does NOT have a www at the front, but most browsers will load the intended site anyway. I'm betting the site owners recently disabled hotlinking (linking their images from outside their site) so when you were visiting the wrong address (with the www) the images wouldn't load.
www. is the default.. neither should really be wrong. If hotlinking has been disabled, they probably just forgot to add the www. version as well as the non www. version as they would likely both need to be put on the allow list.