Hi Drorshalit,

You supposed to follow the below steps,

Step 1: Get into the option Dashboard > Humanity > Widget Area > Widget Areas For Mega Menu there you need to create a new widget.

Refer the screenshot: http://prntscr.com/cp5r19

Step 2: Get into the option Dashboard > Appearance > Widgets > there you can assign the image under the newly created widget.

Refer the screenshot: http://prntscr.com/cp5rnt

Step 3: Get into the option Dashboard > Appearance > Menus > Edit Menus > there you can assign the widget area under the Display widget area option.

Refer the screenshot: http://prntscr.com/cp5s6y .