m.imgur.com/KfVA0vQ?r
I see a lot of images with this dark red star underneath them and I'm curious about what they mean.

I think there's an explanation on the site somewhere, but the stars mainly have to do with rating. However, I think you also get different coloured stars from parent/child posts and flagged posts have a brighter red star.

It means the site considers them high res. Though what measurement the site uses to judge that, I'm not sure, since it sometimes doesn't make sense.