Do you mean gallery photo or outfit? I'm confused because you say "Pixie's sixth gallery photo" but she doesn't have a sixth gallery entry or a sixth outfit :P
2 years ago(+1)
Gotcha :) Okay, so to the best of my recollection:
Jessica's Outfit: Swimsuit. Go on a beach date with Jessica (might need to buy the swimsuit from the shops)
Pixie's Outfit: Set up all the Hallowe'en decorations (same as you did for Jessica and Gwen's outfits) and then just sleep. Might take a few nights, as it won't trigger if other events happen.
Roxy's Outfit: Go back to the bunker after getting Roxy up to Love level 4.
Note that all outfits must be collected in Act Two! Everyone's too busy once Act Three hits ;)
Hope that helps!