That is straightforward! Move FreshBackMac.app contained in the downloaded package to your Applications folder. (Optional) Add to startup applications by going to System Preferences/Users & Groups/Login Items.
How do I use the application?
You would find a tiny icon in the menu-bar (the bar that sits in the top!) which looks like an old-fashioned monitor. Click on that and you are good to go.
I use the pixabay search and it gives no results, what do I do?
This generally means that you are searching for something very specific. Or, the pixabay server may be down, which is less likely. Try searching for wide terms like: "Stars", "India", "Dance", etc. If you still get no errors, report the issue by sending a mail to firstname.lastname@example.org.
I sometimes get alerts like "bad URL", what's this?
This means that the API's the application uses are returning incorrect images, nothing we can do about that. If it is frequent, report the issue by sending a mail to email@example.com.
The options for New Wallpaper are greyed out, what do I do?
This means the application is loading an image. It should timeout and get enabled after 60 seconds. If it doesn't, report the issue by sending a mail to firstname.lastname@example.org.
How do I uninstall the application?
Remove FreshBackMac.app from the Applications folder. Remove the directory and the contents of ~/Library/Application Support/FreshBackMac. All done,... but why?