Unfortunately, Google has not provided this information for the majority of the photos used for Chromecast backgrounds. If you are dying to know more about a given photo, you can do the following:
Many of the backgrounds are for sale on other sites or are not available in multiple formats. If you're looking for satellite imagery you should check out SatDrops.
The purpose of my site is not to violate terms/copyright. If you have a problem with a photo on my site please let me know and I will remove it immediately.
I appreciate the kind words. You absolutely shouldn't feel obligated but if you really feel like saying thanks you can consider buying me a coffee.