I also agree - if consent for each individual (and instructions on how to identity or not) cannot be found, but a photo meets other listed criteria (group shot, public setting, publicly posted, individuals not identified, consent of photographer given, important for fandom history etc.) blurring faces should be a minimum. I don’t think that would detract from the history of cosplay at all, especially given the most-often pseudonymous nature of fandom. Linking to a publicly posted photo seems ill-advised because of the exact nature of where photos can be posted (difficult to determine a rubric of explicitly-fannish posting places that carry inherent “proof-of-fannish-intent”).