Some of my friends running Sierra experience an annoying issue, they cannot change desktop wallpaper. I had this issue before but when I started to use this app the issue remedied itself. So I'm happy with it.
Cool! I have been getting tired of the standard desktop pictures built-in from Apple. I used to be someone that would customize my Mac all the time, but now I don't make enough time for it. Wallcat is easy because you just launch it, tell it to launch itself on startup and that's it! You'll have a funny looking cat icon in your menu bar.
Tip: If launching the app doesn't change the desktop picture right away, quick it and relaunch. That worked for me.