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.
If you don't have the Site Map link, here's another way to get there.
* Login to your PayPal account.
* Click on the "Gear icon (Profile)" top right of screen.
* Click on "Profile and settings".
* Under Products and Services.
* Click on "Website Payments".
* Look for "PayPal Button".
* Click on "Update"
You should see your Saved Buttons.
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.
(Note the suggested graphic image size limits as that is important.)
There you can enter the URL of the desired image you want to use and Save.
Last, the image should be stored on a secure server (https://). Else a web browser may display mixed content messages to the customers.
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.