Where in K-ON! does the second panel of this image of Azusa come from? 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]
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).