We discussed this on IRC, and decided on the following guidelines (approved by albert).
For pictures of character A (eg Konata) cosplaying as character B (eg Archer):
- mandatory: add both character tags (eg "izumi_konata", "archer");
- mandatory: add "cosplay" tag;
- if you have OCD: you can add another tag (in addition to the other ones, not instead of them) with the _(cosplay) suffix (eg "archer_(cosplay)").
For photos of a human cosplaying as character X (eg Suzumiya Haruhi), add the character tag (eg "suzumiya_haruhi"), the "cosplay" tag, and the "photo" tag.
Updated by albert