Perhaps I wasn't very clear in my original post. I'm not the one concerned about security. I'm concerned because visitors to my shopping cart will get the message 'unsecure elements on this page'... since my shopping cart page will be using a header image which is hosted NOT on paypal's servers but on my own unsecure host's server. If I could find a way for paypal to host my header image, I'd do that. But I can't find how to upload my image to paypal. The only option I can see for inserting my own header image onto the shopping cart is to link to a remote image.
Paypal page says: "Header Image URL - Please specify an image that is a maximum size of 750 pixels wide by 90 pixels high. Larger images will be cut to this size. The image you choose will appear at the top left of the payment page. We recommend providing an image only if it is stored on a secure (https) server." There's no upload button, it wants a url.