People put their cosplay pictures online for THEM to show off, not for other people to take and post everywhere as they please. Just because something is online doesn't mean it's free to take and use as you wish. You don't own the picture, the person who took it/is in it does. And yes, they do own it - it's copyrighted.
You really should try and get permission.
I know I'd be pretty damn pissed where I to find my pics posted everywhere, and every single cosplayer I know (and there are a lot) is the same.