Like everyone else recommended you the talkimg.com is the best website that you can use to upload your images and the proxy invalid image error will disappear. That error mostly happens when someone uploads images to Imgur.com and those images won't work on this forum.

You can try another good website which is https://imgbb.com/ to upload images and the image uploaded on that site also work on the forum flawlessly. But, still I recommend you to use talkimg.com only as that's a project made by a reputed member of the forum and that one works best for Bitcointalk.org.