Creating a product that accepts an optional image upload for personalization

Hi – I am working with my high school boosters to sell ads in our sports bulletins. I’d like to modernize this and sell ads to our partners based on how many pages of ad space is available (I can do this with products and ‘manage inventory’ already), but I’d like a place where they could upload their art as part of the either product adding to cart, or on the checkout page.

Because this is a volunteer thing and a fund raiser, I’m looking for something that’s not a paid add on, if that’s possible.

I’ve seen gravity forms, but it is all premium. It seems like there should be a way to do this. Can anyone point me the right direction?