The checkout page should use site relative links to the images so that if the page is requested using HTTP it will use HTTP to retrieve the images, and use HTTPS if the page is requested using SSL.
have you added links to images on the checkout page that are loaded from a different server using a full URL? if so, you will need to edit those links to use the HTTPS address.
if you need more details, it would be helpful to see the problem, if you could provide a link and steps to reproduce.