Don't add any essential oil preservatives to something you'll be using photographically. Using EOs as preservatives is all well and good for things with a food or cosmetic destination, but not here. Sodium benzoate is probably the least-troublesome compound to add (I'm assuming you're doing gum-bichromate?), and I can't think of a reason ascorbic acid would cause issues either. Still, stick with something unreactive and stable like sodium benzoate.