One of the very nice things about cosplaying in Japan is how much relative control we have over our images.
Ettiquette tells photographers to ask to take photos and allow us to pose for them.
But there's a little bit of additional ettiquette you might not be aware of:
It's also polite to ask to post those photos online.
The reason our crappy, puffy-eyed faces and accidental closed-eye snaps aren't all over the web is because the folks taking photos aren't uploading them without our permission. And really, isn't that kinda nice??????
If you've got casual, unposed snaps or videos of people in costume (or out) -- please don't post them online to gallery sites. Personal photos and cosplay photos aren't the same thing... and if you haven't receieved permission to snap the photo: the photo is already in violation of both event and standard cosplay ettiquette. It's one thing to take a few snaps for yourself, but unless your subject knew they were being photographed and had a chance to prepare: those photos aren't to share.
The same goes if you've taken less-than-flattering images of cosplayers. Use your best judgement. If someone looks crappy, has their eyes closed or has a huge mole that you know they cover in all of their own photos: don't put those particular photos online at all. If you look good in a photo and someone else looks awful, crop them out, blur their face or give them some digital help to look a bit more together (cover up that mole for them if you know it bothers them so much).
Sites like cosplayer's archive, Cure and even cosplay.com (to some extent) are designed to showcase your own photos and posed, composed shots of your own groups or models -- not casual snaps of strangers. When uploading images that will be searchable on Japanese cosplay sites ~ upload ONLY posed images of yourself solo or with friends who have given you permission. Other photos are welcome in personal albums and journals but even then should be posted with caution, and if at all possible, permission.
Bad, unposed photos can be a blow to the confidence of even the most composed cosplayer and quite a few cosplayers want to control all images of themselves to avoid trouble at work, with their families or in their personal lives. Posting a photo you've taken of a Japanese cosplayer, even one you're on good terms with, without their consent on a public showcase site can quickly lead to misunderstandings. Most simply said: it's been spread around that such conduct is "bad manners" in the Japanese community.
While in the US, the attitude is very much that the photo belongs to the photographer: in Japan, the prevailing attitude is quite the opposite. Event photos are typically shared with only those cosplayers in them through email or password-protected, auto-time-out albums. It's a huge pain in the ass but it's also incredibly thoughtful.
Please keep this in mind and be as considerate as possible when uploading your event photos, ESPECIALLY to Japan-based gallery sites and cosplay communities.
If you really want to share event and gathering photos on public blogs or web-pages (again, especially ones available primarily in Japanese): it's a good idea to get in the habit of asking everyone in them if you can share their photos online right when you take them to avoid any misunderstandings or hurt feelings. You'll be surprised how many people will turn you down for privacy or insecurity reasons... but plenty will say yes, too ~ especially if you are in the photo or part of a large group who is gung-ho about having the photos to use themselves (no one wants to be "that guy" who renders a photo of 30 other cosplayer unusable for all of them).
Sorry to be a downer
But event photography ettiquette is SO different here that it really needed to be said.
One more note: If you are a teacher of children, especially an employee of a public school board, photographing or videotaping your students in ANY situation and sharing those photos or videos online can cause some major fallout with your workplace and has, in some cases, led to people being disciplined, fired or made targets of investigations into possible ulterior motives. Japanese people are downright ridiculous about the internet and photographs of children. Cosplay is not well-understood to begin with ~ posting photos or videos of yourself with identifiable students with either party in-costume is a great way to risk the wrath of parents, employers and the community. That isn't to say not to take those photos... but control them like the dickens and be sure there's no way on earth anyone in your school will find them and make a stink.