"My goal for the day--" Aside from taking a break from studying, of course. "I'm going to catch you off guard at some point today. When you least expect it, I'm going to catch you on camera, too. Not in a bad way, no." Riddle seems to be thinking through his words carefully, trying to figure out how to say this aloud.

"Your smile. I'm going to capture it."

That could mean so many different things. What Riddle meant by it wouldn't be seen until later, surely. It may be extra confusing to hear such a statement, when Cater smiled in almost every photo he's in, but Riddle seems so sure of himself.