Why can’t I add a custom image in my navigation?

I figured it out.

You have to search your style.css file and find

/* Buttons */

Under that you will find the following classes

.menu-toggle,
article.post-password-required input[type=submit],
.bypostauthor cite span {}

.menu-toggle,
button,
 {}

button[disabled],
input[disabled] {}

.menu-toggle:active,
.menu-toggle.toggled-on,
button:active,
 {}

There are 2 states of the button that you need to configure.

Hover and active.

Just change the background-image:url(''); for each state to what image you want in the background.

Then just either negative indent the text by -9999 or delete the text from the header.php file as you previously stated.