There was a post on the .com boards about this. You can no longer do what you used to do. The system now automatically takes the first picture in your listing and uses it as the gallery image.
Some people used the gallery image as part of branding and put their logo in there. The first image in their listng would be different from the gallery image. You can't do that anymore.
You have two options at this point. Either put the image you want to show up in the gallery as the first image in your listing with all other images to follow or stop having an image specifically for the gallery and go with eBay's changes. You don't have a choice anymore. Sorry.
When you find doing what's right hard to do - you've got a problem.