Your browser reports that it doesn't support modern technologies - in this instance, WebP. This means you're most likely using Internet Explorer or Safari. You can carry on using the site if you really want to, but won't be able to see any images until you upgrade to a modern browser.
Waterfall's recommendation is Firefox - it's fast, secure, and does a good job at protecting your privacy.
Happy 4th anniversary Cat Busters, I'm sorry I don't have an art for it right now, but maybe after my birthday I'll do something