As part of eBay’s Spring announcements they have announced “Free Supersize Picture Option for All Sellers“.
As of 15th April 2014 the option for Supersize pictures will be free. This allows users to zoom in and see details of your images from within the item page.
And if you can’t really be bothered going in and changing all of your listings then come 15th May your listing images will be Supersized automatically if “they’re a minimum of 800 pixels on the longest side.”
So make sure your images are bigger than this to take advantage of this feature.
To be honest when I’m buying this image is more annoying than useful but people must find it helpful if eBay have decided to make it free as part of their highly visual vision of the future for the site.