Need help with Google Photos [View all]

I can't figure out to upload photos from my Android phone to Google Photos. I have a lot of photos in there that I'd Iike to upload to Google Photos, so I can then download them to the Android tablet I do most of my internet activity on.

At one time, my phone was set to automatically backup photos to Google Photos, but it stopped doing that a while back. I don't know how the setting got changed or how to fix it.

Basically, I want to set Google Phtos to backup again, AND I want to upload a bunch of pics that weren't automatically backed up to Google Photos. Can anyone help?