Just did a quick test. Unfortunately, it seems that there is no built-in way to do it. If you want to have images in your navigation, you will need to use JS. You can search on the dev wiki to see if one already exists. If not, you can use the forums there to ask for help creating such a script. Although, I am not sure if such a change would constitute a violation of the customization policy. The wiki navigation is a gray area where you are allowed to make some changes but don't have free reign.

Hello! Just to add, it may not be good for user experience to add an image there as it might make the text hard to read. Best to make the user experience as easy as possible. :)

Using JS to alter that menu is against the Customization policy. However, you can use unicode "images". (Google "unicode images".) Since they are just text, they are allowed.

As Spongebob said though, do not change the text to just unicode images as the menu should be as straight forward as you can make it for a better user experience.