Both your logo and background color are something you can set in your paypal options, and not have to worry about again in your shopping cart code.
Also, you may want to consider using httpS for the logo to avoid your users getting the "do you want allow the non-secure..." on your checkout page. It will instill more confidence that way.
Have a look the order form on
http://www.edevmachine.com. I use both a custom logo, and background. Both were setup in my Paypal account options.