"If you're on pc, then it's pretty straightforward: there's a comment box below the pics. But I suppose you're asking this because you're on mobile. As far as I know, the only way to do it is to use to the beta version of the mobile website. You can access it by clicking the red link on the homepage. Once you're on the beta version, when you open a picture, there's a button on the toolbar at the top that opens the comment section and you can leave your own comments. Besides, the beta version seems to be better overall, although it's still not great compared to the desktop version"