Why is a plain HTML "Submit Order" button showing on the secure checkout page for credit card instead of the custom button/image set as the " Selected Buy-It-Now Button " in  Order, Upsell & Content Icon Button Settings?

The secure checkout page uses the https protocol as a secure page. All content on the page must also be secure (using https:// protocol) in order to avoid warning messages for insecure content on a secure page. 

If you create a custom button/image (under Create >> Custom Buttons/Images) and do NOT select the option  "Use https:// in the resulting URL instead of http:// (for UPLOADED images only)" the system will automatically replace the insecure button/image with a plain HTML "Submit Order" button.

Either edit the custom image and check the option to "Convert this image to use https:// instead of http://", or else create a new version and selecting the "Use https:// in the resulting URL instead of http:// (for UPLOADED images only)" option.