Update 2021: How to do it in the new Google Photos Android app:
- Open Google Photos.
- Click on your avatar in the upper right corner.
- Click on Photo Settings.
- Click on Backup & Sync.
- Click on Back up device folders
- A list of folders will appear with a toggle on the right.
- Toggle on / off the folders you want to back up.
Update 2014: How to do it in the new Google Photos Android app:
- Open Google Photos.
- Go to Settings on the left panel.
- Click on Backup & Sync.
- Scroll down a bit.
- Click on Choose folders to back up...
- Activate or deactivate each folder by clicking the switch.
Google+
From: Back up photos & videos automatically
Turn on or off for individual folders
- Open the Photos app .
- Touch the menu icon at the top left and then On Device.
- To the right of each folder title, touch the cloud icon to turn Auto Backup on or off for that folder:
Auto Backup is on for that folder.
Auto Backup is off for that folder.