1. Upload the desired image within Media > Add New. Ours is 280 x 200. Note the URL of the uploaded image.
2) Next, Go to Appearance > Menus. Before doing anything click 'Screen Options' at the top and click the tick boxes next to all of the options.
3) Now go to 'Product Categories' and add one to the menu. This doesn't have to be a category, it can be a page, or a custom link etc. For this example we'll use a category - Jackets.
Ensure that it is indented under a top level link. You'll see it is pushed in within 'Women'
4) Open up the Jackets link you've just added. Here, we're going to change the title 'Jackets' to 'Amazing Jackets' (if you like). Next, under 'CSS Classes' include 'image-item-title'
If you don't want a title to appear, i.e. just an image, use 'image-item'
Finally, in the 'Description' field paste in the link to the image we uploaded back in Step 1.
5) Finally, ensure that the top level menu (in this example 'Women') is designed to use a mega dropdown. Open that up and add 'menu-full-width' to its CSS Classes field. You can optionally add 'col-5' or 'col-4' etc if you want to change the number of columns to display.
Remember to save the new menu way down at the bottom of the screen. It should look great now!