I find it sometimes takes a while for photos to load (or render), so the square is black until they do. I'm not sure why but I suspect ppl are posting large versions and the forum software limits the size to what we finally see. Even a full sized jpeg straight from a phone will take longer to render than one that is posted at, say, 640 x 640 pixels.
That's just a guess and I've never loaded a photo directly from a phone so I don't know if there are any steps to chose the size of the image. I always upload my photos to Flickr and post a limited-size copy to forums.
How long do you wait for the photos?