I usually use imgbb.com for hosting images, and lately I've noticed the "Bitcointalk.org Image Proxy: Invalid Image" warning pop up frequently. The problem is inconsistent, sometimes the image is displayed fine, other times the image won't load.
I've seen that warning a lot more recently and I think it's just a forum problem.
I thought it might've been due to more images trying to be loaded on the forums end and the proxy/Webserver dropping requests/communications. (it's especially apparent on pages with multiple images but is normally resolved by waiting a minute and refreshing).