Does anyone know if the picture when you trick or treat determine if you're going to be hexed? or Or is it completely random?

Also can I be hexed while I still have one on my HA, or do I need to cure it first? I'm trying to get the hexed achievement (x

I'm petty sure it's just random. Also yeah you can be hexed multiple times in a row, even if you still have a hex from before.