We don't know...
The assumed past members of the Slug Club are;
- Lily Evans
- Dirk Cresswell (now Head of the Goblin Liaison Office)
- Barnabas Cuffe (now editor of the Daily Prophet) (HBP4)
- Ambrosius Flume (now owner of Honeydukes) (HBP4)
- Ciceron Harkiss (HBP4)
- Gwenog Jones (now Captain of the Holyhead Harpies) (HBP4)
- Marcus Belby's Uncle Damocles (inventor of the Wolfsbane potion) (HBP7)
- Cormac McLaggen's Uncle Tiberius (HBP7)
- Lucius Malfoy (HBP7)
- Theodore Nott's father (HBP7)
- Eldred Worple (author of Blood Brothers: My Life Amongst the Vampires) (HBP15)
- Tom Riddle
Only a list of 12, and there are 38 students shown in the photos. We could assume that each of those members are represented in the photos but we do not really know their age nor their physical attributes beyond a select few (i.e. Lily, Lucius, and Tom). Couple this with only being able to guess what decade the photos were taken in based on Slughorn's age (kinda hard to tell) and the clothing styles if we don't have a known member again (Lily and Lucius; Tom doesn't seem to be in a picture).
Even the HP Wikia (which has been known to the members of this community to be unreliable) does not even take a guess at who is in the photos. Captions are just generic descriptions like
"Slughorn and a band of former club-members, inlcuding Lily Evans."
"Slughorn with four girls and three boys, likely between 1930 and 1950."
So without a direct identification from cast or crew of the films I don't think we have enough information to go on.