Please note that using the Insert an Auctiva Hosted Image tool does not require the use of the individual URLs for images as you would if you were using externally hosted images, it does allow you to select from images already uploaded into your Auctiva account in a manner very similar to the image selection area.
Please also be aware that as a result of eBay's changes over the last two years our old templates became non-viable for use on eBay, which is why we have released the new suite of templates. Most of the older templates were created with older versions of HTML and many contained active content which is no longer allowed on eBay. As such, we have the new templates available which are fully compliant with eBay's current requirements and contemporary browsers. We suggest that the older templates no longer be used to avoid difficulties.
I understand what it is you are looking for and unfortunately it is not as simple as replacing a current image tag from one of the new templates with a tag such as you have mentioned here; the custom template editor was also changed to accommodate the current templates. As the new templates were designed to have the images supersize, you would need to modify the CSS on a per image basis in the Source from within the template editor to prevent the ability to enlarge the image. You should be able to customize one of the new templates and in the code for each image remove the tag:
<label class="card-gallery-img aspect-ratio aspect-ratio-4by3 cpointer dblock" for="lightbox-1">
which will prevent the images thus modified from supersizing when clicked.