Is one absolved of responsibility by posting a picture of a borrowed pen, whose owner is unknown, in the college group three times, despite feeling embarrassment, knowing that several questions appear in the group which might reduce the picture's visibility?

Taking a pen from its owner for use and then returning it is considered an 'ariya (a loan for use), and an 'ariya must be returned if it is still extant, and guaranteed if it is damaged.

Its owner can be identified by asking the students, and if it is not possible to identify him, then it can be given as charity on his behalf.