Irvue lets you keep your Mac screen looking fresh with Unsplash photos. You can set the application to change the wallpaper automatically every 3 hours, 12 hours, 24 hours, week, two weeks, or one month. You can select the same or different images on your displays.
I like it when it works. One external monitor of 2 - background screen flashes up to a nice pic, then after 2 seconds, turns a yuk blue