To set a custom backdrop, the best way is to upload a Photo to an album on your Google Photos account.
Chromecast allows you to select custom albums from your Google Photos collections and display them as part of the Backdrop
Upload Phones to your Google Account
- Go to photos.google.com
- Log into with your Google Account
- Create an Album and upload your Photo
Sign into your Google Account
- Open the Google Home app on your mobile device.
- In the top left corner of the home screen, tap Menu .
- Tap the triangle to the right of your name and email address.
- Tap Add account > Google.
- Type your full email address.
- Type your password and tap the right arrow to sign in.
- Open the Google Home app .
- In the upper right corner of the home screen, tap Devices to see your available Chromecast devices.
- In the top right corner of the device card you'd like to set up with Backdrop, tap the device card menu .
- Tap Backdrop Settings.
- Tap Yes, I'm in to agree with Backdrop Terms and Conditions.
- Then select Google Photos under Backdrop preferences