I recently read through the novels that were translated on baka-tsuki, which made me aware of some issues with this series. The Sword Art Online series ends up taking place through a few different games (Sword Art Online [SAO], ALfheim Online [ALO], & Gun Gale Online [GGO]), and the appearance of the avatars used by the characters can change between the games. There might be a need to differentiate between which game (or perhaps novel volume, though that would be less ideal) the depicted character is from.
There is also the issue with using the character's names, as they have both an avatar name and a real life name. In most cases the avatar and the real life person will have an identical appearance if they're a Sword Art Online player (outside of perhaps hairstyle and color, though eye color was also changeable), as their real life appearance was forced upon their game avatars (episode 1 and volume 1). Kirigaya Kazuto /Kirito ends up with a trap appearance in Gun Gale Online in volume 5. Two non-SAO players where this is an issue is Asada Shino/Shinon and Lyfa/Lyfa's player (though there are currently no images on danbooru depicting her real life appearance, but it may become an issue as the anime progresses past episode 12-13, assuming the 25 episodes listed on wikipedia and AniDB is accurate). Additionally our current tagging has used the game handle name for all the characters, but for the character "Asuna" we're using her real life name, "Yuuki Asuna" for the character tag.
Current Characters with issues:
Asuna (Real name: Yuuki Asuna, revealed at end of volume 1 which would equate to probably around anime episode 12-13): Real Life/Sword Art Online (post #1122700), ALfheim Online (post #1137436 one on the right)
Silica (Real name: revealed at the end of volume 4 which probably equates to episode 25): Real Life/Sword Art Online (post #1142517), ALfheim Online (post #812763) (more or less the same, though change in outfit and now has cat ears and tail).
Shinon (Real name: Asada Shino, revealed early in volume 5, same volume the character first appears): Real Life (post #1120998), Gun Gale Online (post #1152417, post #1152420), ALfheim Online (post #1025327, 3rd from the left).
Characters with potential issues in the future:
Lisbeth (Real name: revealed in volume 2, probably equates to around episode 6, same volume the character first appears): Real Life (w/o pink hair)/Sword Art Online (post #1210439), ALfheim Online (baka-tsuki image more or less the same, slightly different outfit and presence of pointy ears).
Shorten version: There is an issue of lumping all character depictions under one character tag, though the appearance of that character can vary between real life and the various avatars they might use in games of the series. There is also an issue on which name is used and when that name should be applied, right now almost all the characters are labeled under their game handle name regardless of how they're depicted and in the case of "Asuna" the tag being used is "Yuuki_Asuna" which is her real name and somewhat of a spoiler (it reveals the fact she didn't use another name in the game which wasn't revealed until the end of volume 1, likely episode 12 of the anime).
Updated by z905844