You can set a different photo cover to match each individual gallery you have!
Click on the Event you’re working on, and then click on Galleries at the top.
Click “Edit" and then, click on the “Cover” tab.
To change the cover photo, click on “Change Cover Photo”, choose the photo you would like it changed to by dragging from your computer or browsing photos from your files, and click on “Publish” or "Update".
Your cover photo will now be changed and will applied to all future web gallery visits and emails you send out for that gallery.