If you want to change your desktop wallpaper, but you don't know how, keep reading.
First, go on "SAFARI", and search "(Whatever you want) desktop wallpaper". Screenshot (Command-Shift-4) the picture you like. You can close the tab after you screen captured the picture. Click on "SYSTEM PREFERENCES", the icon next to the trash bin, then click on a button that says "DESKTOP & SCREENSAVER". Scroll down on the left bar, until you find a folder named "DESKTOP", click on your screenshot, and it'll change!
If you have any questions about your iMac, you're free to comment them down below.