1.To change background image, go to WordPress Menu > Appearance > Arcade Themes Setting > Background
2. You will see on your left side, the image of the background and you can click change image.
3. You can take the image from your media library and you also can upload files from your desktop. Then, click choose image.
4. It will change immediately the background of your website.
5. Besides that, you also can choose how you want your background to look alike.