1. Look at the person who posted a picture just above you.
2. Think about that person's personality. Are they happy? Sad? Lonely? Perverted? Mischievous? Naive? What are their interests or hobbies? Do they have any unique characteristics?
3. Post a picture that captures an aspect of their personality.
So, for example, let's say our beloved furry administrator posted above me. I might post something like this...

1. After posting a picture, you must wait for two more people to post pictures until you can post one again. This is to prevent two of our members from going back-and-forth.
2. If you're just commenting without posting a picture, place your comment in double-parenthesis. ((Like this))
3. Pictures must be from anime, manga, or something related to either one of those. No real life pictures!
4. If a picture is too wide, resize it to around 600 pixels.
5. As usual, keep it PG-13.
6. A fair amount of good-natured teasing will go on in this topic. If you think you can't handle that, please don't post here.
7. On a related note, there is a difference between teasing and insulting. Post here in good faith, and don't be mean spirited. If you think a comment might be over the line, it probably is.
6. ANIME AND MANGA PICS ONLY! This can include doujins, fanart, etc., but no more Futurama or image macro pics please.
7. Since there's been some confusion, here's a new rule: when you post a picture, you must explain exactly what aspect of a person's personality this represents. Make your explanation fairly detailed... at least three or four sentances.
8. Try to avoid a specific experiences... for example, just because someone has had computer problems doesn't mean you should post a computer pic. Instead, focus on personality traits, habits and hobbies. For example: I took test yesterday, and I enjoy building Gundam models. Posting a picture of me taking a test would not be good, since that's a fairly common, mundane and unremarkable experience. A picture involving Gunpla would be good, since it revolves around a hobby of mine.
If you have trouble finding images, an excellent resource is Danbooru. You can search via tags there, exactly the same way you do on Sankaku Channel. Beware, the site is NSFW.
That should cover everything. Now, start posting!