You can add your logo the following way:
1) go to your paypal account
2) select Profile
3) Custom Payment Pages
4) add Custom Page Style
5) add header image URL with your logo
6) save your style page
7) make your new style page as primary style page
Make sure you have ssl in your store and the image link of logo is in SSL, or else it will give warnings of unsecure contents when a customer is in paypal site for payment from the oscommerce store.
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