Oh, I see. Depending on the exact version of the ChromeOS that you're running, one of these methods should work for you:
1. Right-click anywhere on an empty area of the desktop screen and in the pop-up menu, click Change Desktop Background. This should bring up a new window that will allow you to change the background image.
If that doesn't work, try this:
2. Click on your account icon in the lower right corner, select Settings, and then in the Settings menu, under Appearance, select Set Wallpaper. This should bring up a screen with several built-in wallpaper images and you should also be able to click on Custom and add a new image of your own.
If somehow neither of these methods works for you, then just let me know and I'll continue to work with you. But if this did answer your question, please use the rating box at the top to rate the answer at least 3 Stars/OK Service or better so that I receive credit for assisting you.