Google Photos Auto-Backup Exclude folder
You should be able to select folder settings in the photos app. Per support.google:

You can choose to automatically back up your Android's photo and video folders, including photos you've downloaded, screenshots you've taken, or photos saved by other apps.


  Turn setting on or off Open the  Photos app > touch the  menu
  icon/button  > Settings > Auto Backup > check or uncheck Back up local
  folders
  
  Turn on or off for individual folders Open the  Photos app > touch the
  Photos menu icon at the top left > On Device > to the right of each
  folder title, toggle the cloud icon to turn Auto Backup on or off for
  that folder