You do not have to keep the engagement secret. Dinner at a restaurant to celebrate an engagement is not generally a problem, but the issue I see from what you described is having photos of rings put on your fingers. USCIS does not know if that is an engagement ring or a wedding ring and there is NO way to prove you aren't married. I would absolutely not include photos that involve rings, period.    Having a casual dinner to celebrate getting engaged is not the issue, announcing the eng