Make the changes and then set it as Mac’s desktop picture.
Pro Tip: Wish to crop, brighten, or edit the image before setting it as a wallpaper? Double click to open the picture inside the Photos app and click Edit. Click the share icon at the top right and choose Set Desktop Picture.Next, click to select the desired picture.You will find it in the Launchpad, Finder → Application, or you may press Command + Space Bar and search Photos.
How to change desktop wallpaper from Photos app