Yes - you can use the Android app to cast photos to the Shield.
This is the app to use. https://play.google.com/store/apps/details?id=com.furnaghan.android.photoscreensaver
Hmm I would think you could as long as you can get the App on the device.
But the photos don't work as well as they did when Google made it for TV. The photos and screensaver app works quite well.
I like PhotoGuru. It can also display photos from my DLNA server. https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser
No built in way and it's fucking embarrassing.
Apple TV a decade ago could.
There is a paid app, I think called gallery and screensaver, that I thought the purchase was worth it. I have it set to certain g photo albums as a collage screensaver. It also has just a plain gallery mode, but I don't really use it.
I've never tried side loading g photos, but I imagine a NODPI arm 8 .apk off of apkmirror.com could be tested.
[Edit]. I just saw in the Google photos update I got today (6/26) has a feature called frames and it lets you display photos on the shield from your phone!