Using the pmpro_paypal_button_image filter to change the PayPal button image on the PMPro checkout page.

my_pmpro_paypal_button_image.php

<?php
/**
 * Change the PayPal button image on the Paid Memberships Pro - Membership Checkout page.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_paypal_button_image( $url ) {
	return 'https://www.paypalobjects.com/webstatic/en_US/i/buttons/checkout-logo-large.png';
}
add_filter( 'pmpro_paypal_button_image', 'my_pmpro_paypal_button_image' );

This recipe is included in the blog post on "Change the PayPal Button on Membership Checkout" at Paid Memberships Pro here: https://www.paidmembershipspro.com/change-the-paypal-button-on-membership-checkout/