Turned out to be an nginx config issue.
Solved: by adding a rule to my nginx config to forward requests with URIs starting with /content/images/size
sudo vi domainName.me-ssl.conf
# Rule to forward requests with URIs starting with /content/images/size to the Ghost process (or any other backend process)
location ^~ /content/images/size/ {
proxy_pass http://127.0.0.1:2368;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
}