Yeah, you have to send them a picture of yourself when you sign up, like you do for a passport or a drivers license.
It gets attached to your account and when you get on the boat you are given an ID card to use to make onboard purchases and to disembark from the boat. Every time you leave or get on the boat they make sure the photo on your account matches who you say you are.
Is this a post-COVID thing? I've been on about a dozen cruises, most on Royal Caribbean ships, and they have always taken a photo at check in. I've never submitted one in advance.
I submitted a photo in 2018 as well, probably as part of an online pre-registration. I think during this particular trip the lady also retook our photos with her tablet.