Email attachments.
websites
Contact lists
Passwords
Photographs
Photographs with encrypted messages
http://www.makeuseof.com/tag/4-ways-to-hide-secret-messages-in-pictures/
Yeah, I used to buy a lot of code in an image (commercial purposes). We didn’t have anything evil behind it, but there was a lot there.
I tell people, just because you think you see a white page, doesn’t mean there’s not a whole lot more going on there.