Two days ago, my Instagram posted some obvious scam photo about Ray Bans on my account. When I found out, I took the picture down. I changed my passwords on Facebook, Instagram and my two email accounts (All passwords rated strong by Google; the previous password was also strong). I also added two factor authentication on Instagram. You'd think this would do the trick, but today the same photo was posted on my Instagram. What can I do? Should I just start new accounts for email and social media? There's no record of any sign-ins on other devices. Google shows no suspicious activity. I feel I'm dealing with a dangerous foe. The only place I can think of this coming from is my Microsoft email. I opened a message from a friend that looked suspicious on Outlook, but I didn't enter any passwords or click any links.
Based on the limited info your provided it sounds like your device itself is compromised and keylogged (that or some shadow program has access to post on your insta through an API). Have you tried resetting all passwords on a secure device, then not logging in with the potentially compromised device for a few days to see if it still occurs?