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