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.
In the Insert Gallery Window, located here [green icon] in your text edit toolbar:
A pop up window will appear, go through the steps of adding a gallery, and right before you click ‘save’ to add your gallery, the last option will be [optional] to ‘Sort or Exclude Images’, see:
After you have selected your images to exclude, click ‘save’ and then publish/update your page.