I have noticed that if a customer adds an image that has a price discount to their cart, that the discount will stay even if the discount is removed from the image.
For example, try adding a discount of 50% to an image, then login as a test customer, add the image to the customer's cart, go back to being an admin and remove the discount, and then go back to the test customer and you will notice that the image in the cart still has the discount.
I would think that this is not the way that it should work, since most discounts would only be offered for a specific timeframe. For example, I was thinking about doing something like a Labor Day sale. But if a customer adds a discounted image to their cart, it will stay on discount indefinitely (which is not what I intended).