Image in menu navigation

That is a major undertaking that falls well outside the scope of what the theme is meant to do. You would have to change the Menu walker class and make some significant changes to the menu styles.

To get you started, read up on the Walker classes in WordPress: https://codex.ww.wp.xz.cn/Class_Reference/Walker#General_Menu_Example

I’m closing this issue as it falls outside the scope of theme support.