Image links not displaying properly.

I'm trying to figure out why images (posted to imgur) that I link to my thread are often not showing up.

I think that's something with the image, as if it doesn't show once, it seems to be broken forever.
Some images work, but most of them don't.

They work perfectly fine in the post edit window, but when I apply the changes / post the comment, they turn into that red "X" and stay that way.

They're 1920x1080 PNG image files (if that matters).


I don't want to link the specific thread, as it could be considered spam-advertising. It's in my signature though.