How to remove or change the PrestaShop PayPal logo
To remove the PayPal logo from the right column of your store's frontend, log in to the backend of your PrestaShop, click on the Modules tab and then on the Positions sub-tab. Find the table Right column blocks and in it you'll see listed an item called PayPal. Click on the "delete" button to remove the logo from the position.
To change the PayPal logo, upload the new image to the folder on your hosting account where the PayPal logo is located (e.g. public_html/prestashop/modules/paypal/img). Then rename the default image using any name you want, and rename the new image so that it has the name of the original logo (the name of the original image is probably vertical_US_large.png).
For screenshots and more details check out the tutorial on managing the PayPal logo in PrestaShop.