yeah I haven't used Rembrandt theme but in Davinci you just need to add to the css style in Customization > Head > Css Style >
.b-add_order__btn:after {
content: url('
http://YOURDOMAIN/URL/safe_checkout_badge-300x56.png');
display: block;
zoom: 90%;
margin-left: -5px;
margin-top: 5px;
}
Now my image was 300x56 so I reduced it by 10% because 270px is just about right and fit right under the button. It was off a bit on the left side so I added a -5px margin.
The css class is ".b-add_order__btn" and I used a pseudo :after to attach the image to it. You can play around with it and see what you get. I think you can do similar changes in Rembrandt easily. The good thing about this change is that it will remain working even after updates.
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