When adding a gallery to a page or post, you have the option to exclude a select amount of images from appearing live on your site for that gallery display.
Navigate to the Insert Gallery Window ('Add Gallery' button) in your post/page edit toolbar:
A pop up window will appear, go through the steps of adding a gallery, and right before you select to insert your gallery, the last option will be [optional] to 'Sort or Exclude Images', see:
After you have selected your images to exclude, click 'Insert Gallery' and then publish/update your page. 🙂