I can tell you that’s Alex Hirsch on the stage, who fought tooth-and-nail against Disney just to let him have two characters who are Definitely Just Roommates in Gravity Falls.
Don’t know when it was taken, though. He’s had a lot of similar slides of demonic Mickey Mouse at several different panels over the years. It might have been at a convention in Mexico a few years back?
edit: found it. It was at Festival Pixelatl in Guadalajara three years ago.
Especially your sandals.