The CFAA is terrifyingly broad, and this isn't even a tenuous application of it. Accessing and using someone else's account without their knowledge or permission is a clear violation regardless of who they are or what you do.
Posting "I forgot to log out!" on someone else's account is probably a criminal offense. Posting a picture injurious to their career and reputation clearly violates multiple sections of the CFAA.
Prosecutorial discretion is the only thing rescuing this situation; literally any internet operation on an account that isn't yours is arguably a criminal violation.
If someone gave me access then I'm allowed but am I committing fraud?
I agree that a suit or damages would be unlikely, but that's only at the discretion of judges; the basic CFAA violation here is unambiguous.