PayPal requires sites using their checkout service to include an "official" logo from PayPal in their pages. You can't host this image yourself -- you have to embed the image hosted on PayPal's servers -- which means that everytime someone loads such a page PayPal will have a record of their visit. Here is their justification for this:

Rather than storing the button graphics on your own server, use the PayPal-provided image paths for the graphics to reassure your customers that the checkout is secure and that you are displaying the most up-to-date logos from PayPal.