It seems like there should be a tag for pics like this one, where a girl has fallen down on her butt. It's a common enough pose in anime, but I don't know what the tag is.
gunner, wakkawakka comment #> Good suggestions, but neither tag exists already.
LexPeters comment #> Quite possibly. I tend to get images from Japanese imageblogs, where there generally isn't credit information, so I rely on others to tag such things.