If you're using the app on an Android device you have the option to add content from Google Photos.
In the day's screen tap on the big blue + button to add your snippet and select Photos.
We would love to give our iOS users access to their Google Photos libraries and we're looking into making this a possibility in the future.
You can still use Google Photos as a backup tool for your media files. You'll just need to download videos you want to use locally for 1SE to see them.