The documentation page for Add image to sidebox or centerbox explains where to store additional images for a template, and how to tweak the CSS which renders the images after you add them, but not how to add them in the first place.
At first glance, the likely possibilities are a banners box and a custom sidebox element. I am trying to recreate the look of a Barebones template with two logos (one landscape format in the header wrapped in a <div id="logo"> element, and one portrait format in the sidebar wrapped in a <div id="logosideboxContent"> element) in Responsive Classic (PHP version: 7.4.33, Zen Cart version 1.5.8a).
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