I'm having a similar problem in Zen Cart 1.3.7. I have a custom style set up in my Paypal account, but when I enter the style name into the module in the Zen Cart admin I don't see my logo on checkout.

When I preview in Custom Payment Page Styles in the Paypal admin, I can see the logo, so I know that the URL is good. The url is to a file on the secure server, so that's not the problem.

I don't see "MODULE_PAYMENT_PAYPAL_IMAGE_URL" anywhere in /includes/modules/payment/paypal.php. I do see 'MODULE_PAYMENT_PAYPAL_PAGE_STYLE', but it isn't commented out.

Any ideas about what else could be causing this?