One option is to go to the "My Saved Buttons" page. There's several ways to get there, here's one:
Assuming you have a "Business Account" (not a Personal Account as it does not support Logos.)
Login to your Account.
Click on "More" (top menu).
Click on "Site Map".
Under Tools, look for "Manage PayPal Buttons" and click on that link.
On the "My Saved Buttons" page, look for the "Related Items" box.
Click on "Manage Checkout Page Styles".
You should be on the "Customize Your Payment Page".
Assuming you don't have a custom page setup, select "PayPal" and click on "Change".
On the next screen, look for the "Logo Image URL and Header Image URL" options.
There you can enter the URL of the desired image you want to use and Save.
Note, the image should be stored on a secure server (https://).
If you're not using a secure server for your site, there are 3rd party services that can store the image for you.
For example, check out this one.
