Hi,
If you used an item custom field : when your customers will attach their images, these images will be linked to the products that they have added to their cart, so the only way to upload images will only be to add a new product to your cart.
If you used an order custom field :your customers will only be able to attach their images during the checkout workflow
So I don't think that you'll be able to do what you want without some complex code customization which will require development skills.