This problem only occurs with Firefox. It can be tested with two ways: Upload a big picture. Try to insert it inside the editor. Error message in the console: Code: Image corrupt or truncated Try to open this big picture directly in a new tab - Message in English should be something like "This image can't be displayed because it has errors" Tested on local & on a live website. I'm not sure if the suggested solutions here can be integrated to XenForo. P.S: the tested images size were about 1900x1500.