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 hook has been fully removed in v3.2. The PayPal button is now a fully accessible, text-based button that can be modified via a gettext function filter, custom CSS, or the pmpro_checkout_default_submit_button filter.