There is no way to select specific personal photos for Chromecast to display in Ambient Mode.
How Do I Put Pictures on My Chromecast Backdrop?
By default, Chromecast displays a slideshow of featured images from Google whenever you arent casting anything.

This is called Ambient Mode.
The background images displayed during Ambient Mode fit into several categories, like nature, art, and landscapes.
Since no single image stays on the screen for very long, this feature acts as a screensaver.

Heres how to change the background images on Chromecast:
Open theGoogle Home app.
Want to customize a selection of Google-curated images?Skip to step 11instead of tapping Google Photos.
To use a selection of your best photos, tapRecent highlights.

If you prefer photos of people, continue to the next step.
To use photos of people, tapFamily & friends.
Tap thepeopleyou want to include in your slideshow.

Your Chromecast will now display photos of your selected family and friends during Ambient Mode.
If you want to select from Google curated images instead, tap theback arrow.
Scroll through the options and tap the ones you want to remove.

Tapping a category will remove its blue check.
Only images with blue checks will be displayed in Ambient Mode.
When youre satisfied with your selections, tap theback arrow.
Ambient Mode will now display the images you want on your Chromecast background.
If you want to adjust additional Ambient Mode configs, swipe up to scroll down.
TapHideunder Personal Photo Data to hide personal information in Ambient Mode.
TapLive Albums Onlyto use just your live albums.
Tap a display time to change your slideshow speed.
How Do I Get Chromecast to Show My Pictures?
However, there is no way to have it show specific pictures.
The selected photo will then appear on the TV or monitor your Chromecast is connected to.
Where Do Google Featured Photos Come From?
Google gets their featured photos used as Chromecast backgrounds and elsewhere from a variety of sources.
When Google+ was still active, popular images posted to Google+ were commonly featured.
Google also requests images from time to time.
Before Google features an image, they communicate with the photographer for permission.