I would go this route, tbh. Make it display adds on the free version. Add a timer to show the ads to the user for enough time to get the money. say to the user he would have received the image in x seconds if he payed for the subscription. Try to make the subscription tier based, so it is easier to onboard users to the cheapest tier, and when they have tried your product well, and know the subscription is worth it, they will feel more tempted to upgrade.