RENAMING IMAGES - go to the Image icon on left - 3rd from bottom. Click SHOW MORE and you will see all of the images I posted.  Click on any one of the circles with the 3 dots to get more options.  RENAME is here.  I renamed the first few in there so you can see what I did.  You can keep or change any names on any images.

ADD/DELETE IMAGES TO GALLERIES - go to the gallery you want to make changes to in Edit Mode.  In upper right hand corner click on MANAGE GALLERIES.  From here you can add images. 

SAVE YOUR WORK  If you don't PUBLISH or SAVE and leave a page - you will lose your work.