Moreover all his shared images links to https://postimages.org/
Upon clicking on image, it should enlarge within DDF but it opens the image hosting site on newtab.
Why it is happening:
Postimg/ImgBB links to their site, so Image code is like this: [URL='https://postimages.org/'][IMG]https://i.postimg.cc/Df1qNTwk/Screenshot-2019-04-02-22-40-47-887.jpg[/IMG][/URL]
Where as usual image sharing code should be like this (without URL tags): [IMG]https://i.imgur.com/9EjAehg.png[/IMG]
I suggested him to use Imgur already as it preserves images for long time and doesn't compress but he didn't understood!