Does not appear to be official art. The character line art looks traced and it was obviously pasted onto a different background. Could be based on a screen cap.
This is a derivative work based on a drawing Kakifly made that is in the second volume of the K-on manga. The poses are identical but there are still a number of little changes such as Yui's hair and Mio's eyes. Also Mio's chest is flatter in the original.
