If the OP was in Europe he could be sued by the trespasser due to the GDPR for posting the trespasser photo on a website without his consent.
Being in the background of someone else's holiday photo isn't a problem, but you can't just post publicly identifying photos of people where they are the subject of the photo if you do not have consent.
[1] https://www.ifolor.ch/en/inspire/image-rights-in-switzerland
But seriously, that's not how GDPR works, people are using it as a replacement term for privacy. GDPR is aimed at companies, not individuals running a blog. You could still sue just under regular civil law.