Or you could hard-code it into the sidebar template:
Michaelangelo: _sidebar.php (cart/cart1/tpl/_sidebar.php)
(you can find it by going to Appearance>Editor in your WP dashboard.
Make sure you make a backup in case you mess it up
i added this when i saw your post, here it is at footer --- https://tiaremarket.com
I downloaded stripe icons and edited one of them to height of 30px (width is relative). File: powered_by_stripe.png (used Photoshop CC)
I used ftp to upload this to .../wp-content/themes/davinci/img/css/powered_by_stripe.png
Then I added this code to the css file (I had to create two css entries based on what is used by the theme -- "partner1" and "p-stripe")