[ThePhysicist]

Yearly subscriptions with pre-payment are great for your cashflow, you should therefore always try to get your customers to sign up for a yearly plan. We e.g. offer a risk-free refund policy for that reason: If you're unhappy with the product you can cancel at any time during the yearly subscription, we will then retroactively charge you the monthly price and refund the remaining balance (I understand this might be difficult in your case). We offer our yearly subscriptions at a 20 % discount and most of our customers choose those.

[micropoet]

But in my case, people always can see what they are getting after payment. As they can see the preview also they'd know the dimension (e.g. 4000*2000px, or vector)