When you use a background image for submenu, you’ll see loading submenu, not a good experience. We can preload images while the page is loading.
There are two ways to preload images: 1. use CSS; 2. use JS. You can choose one according to your situation. If your application is based on JS or with lots of JS, no doubt, the second is your choice, otherwise the first is better. While JS is disabled, most browsers still work well with menus of pure CSS.