@giacomosilli In Ghost themes, you can set whether the {{img_url}} handlebars helper outputs relative or absolute urls. By default feature image URLs are relative, IIRC, but that can be overridden like so: {{img_url feature_image absolute="true"}}. What theme are you using?
@giacomosilli I’m not sure this is related, but IIRC {{feature_image}} has been depreciated in favour of {{img_url feature_image}}. You could run your theme through gscan.ghost.org to confirm.