I know that the image is of Azusa from K-ON, but I don't know where it is from exactly. I don't think it was in the original anime?
I'm referring to the second panel of this image [NSFW]
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I don't think this was put in the original anime
lol of course not
It's fanart by an artist named くまもとこ / "kuma motoko" who used to be on pixiv but has since deleted his/her account. Some remnants of his/her art, not including the Azusa picture (but including a different picture from the same set), can be seen on Twitpic (NSFW).