Harry notices the resemblance to Sirius, or Slughorn happens to be naming people in a picture with Lily

Regulus and Lily in the same picture...?

edit: I get that they might both have been in Slug Club but somehow I can't see them getting near enough to be in the same frame.

I wondered if they were going to take the Quidditch photo, with Regulus as Seeker, that Harry and co. find in Deathly Hallows and slip it in here instead.

and we get a quick explanation that Regulus was Sirius' brother and died young at Voldemort's hands

Since they completely skipped that in OotP where Sirius tells it to Harry, that might be an idea...