Hi Tim,
You can specify the image in the
CMS Desk -> Tools -> E-commerce -> Configration -> Store settings -> Checkout process (or in Site Manager -> Tools -> E-commerce configuration -> Store settings -> Checkout process).
Each checkout step has property called Image file name, where this property should hold name of the file used in the checkout process header. The image must be stored in the following folder:
~\App_Themes\<stylesheet name>(i.e. CorporateSite)\Images\ShoppingCart
You can also take a look at the Developers guide:
Checkout process.
Best regards,
Martin Danko