When Does Google Photos Choose To Backup My Images So I have downloaded the Google Photos app and selected free unlimited back up plan.

It seems to display a recycle bin next to images that haven't been backed up.

Now could someone explain how and when the app chooses when to back up photos?

Do I have to have the the application open to backup photos or not?
Is there a way to force sync this on my computer maybe?

It seems very inconsistent and there seems to be no sync button that I have found.
The Mac desktop uploader actually works really well in the background!

I leave the iPhone app open and let it upload everything periodically although it does seem to upload when I leave the app open in the background.