Well, I went to eBay and checked out the "offical" blurb on Gallery and it say's this. Clear as mud to me. Maybe just too much reading lately.
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Adding a free Gallery picture to your listing
Using eBay Picture Hosting:
Include at least one picture in your listing. The first picture becomes your free Gallery picture.
Note: You may change your Gallery Picture as long as your item has received no bids and at least 12 hours remain in your listing. Learn how to Change Your Gallery Picture.
Using self-hosted pictures:
Type or paste the Web address of your picture into the box provided on the listing form. Supported picture formats are JPG, GIF, PNG, BMP, and TIF. All formats are converted to JPG when saved on eBay.
Note: The recommended size for a self-hosted picture is 1000-1600 pixels on the longest side. Source pictures with smaller dimensions can be used, but may result in lower quality Gallery pictures.
Include at least one picture in your listing. The first picture becomes your free Gallery picture.
Note: The picture you submit when you add Gallery is automatically sized to 80 pixels on the longest side in List View and 96 pixels in Picture Gallery view. Gallery pictures are usually generated within minutes of being submitted, but may take up to several hours at peak times.
See the Listing Upgrade Fees section of the Fees page for more information on the cost of listing upgrades for Gallery Plus and Gallery Featured.