The app shows the main product image by default and only switches to the variant image if it is selected using the variant dropdown.
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In cases like the one this merchant describes (when they are upselling only a single variant), merchants sometimes expect that variant's image to show instead.
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You can fix this by overriding the image for a specific offer like this:

#nudge-offer[data-offerid="YOUR_OFFER_ID"] .product-image-wrapper img{

background-image: url(YOUR_DESIRED_IMAGE_URL);

object-position: -99999px 99999px;

height: auto !important;

background-repeat: no-repeat;

background-size: cover;

padding: 0px;

}