I also just came across this on eBay.
You may change your Gallery Picture as long as there are no bids and more than 12 hours remain on your listing.
Note: You must include at least one picture in your listing for your free Gallery Picture.
To change your Gallery picture
If you have provided only one picture, replace the existing picture or add a new picture to your listing.
If you have provided several pictures:
Go to My eBay (you may be asked to sign in) and click the Selling link under All Selling in My eBay Views. The list of items you are currently selling appears.
Select the item you want to change the picture for and select the Revise link from the pull-down menu under the Action column.
In the Bring your item to life with pictures section:
If you selected eBay Picture Hosting, select the picture you want to use as your Gallery picture by moving it to the first (far left) position.
If you selected Gallery picture Web address, (self-hosting) enter the Web address for your picture.
Click the Change button.
Review your new Gallery picture and then click the Save Changes button