Change Payment Icons on WooCommerce Checkout

Here is an example of how to change the default PayPal icon on the WooCommerce checkout page. Live example:

<?php
/**
 * Add custom gateway icons
 * 
 * @param  string $icon       Image HTML.
 * @param  string $gateway_id Gateway ID.
 * 
 * @return string
 */
function custom_wc_gateway_icons( $icon, $gateway_id ) {
	// Example for PayPal:
	if ( 'paypal' == $gateway_id ) {
		$icon = '<img src="' . WC_HTTPS::force_https_url( 'http://your-site.com/image.png' ) . '" alt="' . __( 'PayPal' ) . '" />'
	}
	return $icon;
}
add_filter( 'woocommerce_gateway_icon', 'custom_wc_gateway_icons', 10, 2 );

For a complete WooCommerce setup guide, see Create a Virtual Store with WooCommerce.