Do you mean the "gallery image" that you can choose for a listing when you post it? This image will show up in search results or when a buyer views your "other listings for sale".
To turn this on, make sure you have added images to your saved listing but selecting the "Select an Image" box and then choosing one or more images.
Next, in this same area, you will see a dropdown box that is labeled "Gallery Image". Make sure to select one of your images here.
Finally, in the "Marketing Tools" section of the lister page, check the box that says "Gallery". You'll then see the green text appear letting you know this feature costs 35 cents in eBay fees.
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