I didn't see this in the first two pages so excuse me (and don't send me to time out PLEASE) if it's already been posted
I was lamenting the fact that I have to use 2 black 2x2 pixels and choose that as my "photos" for Chromecast because of the huge amt of data downloaded each hour (like 15MB/hour). That doesn't work well with satellite internet but at least there was a workaround.
Some kind stranger pointed out that in the Google Home app, Chromecast now has an (Experimental) option you can enable called "low bandwidth".
"...and the experimental sources are Facebook and Flickr. The latter also show a low-bandwidth mode that only displays images already stored in your Chromecast to reduce data usage."
"A low-bandwidth mode has now been added so that you don’t have to worry about it downloading too many high-resolution images."
You can Google it to find more info. It was introduced in July but I'm not sure when it was acutally available. And something else I just thought about, I wonder if it works with all versions of CC.
Hope this helps!
Good job posting this! I was going to a few days back but totally forgot. This "low bandwidth" feature is now available in a recent Google Home update and seems to be working for me. With the update it also looks like the 1x1 pixel fix no longer works. They probably set a minimum file size for Backdrop images.