There are two shortcodes available for creating the Thumbnail Mega Menu in SaasLand.  The mega_menu_thumbnail and mega_menu_thumbnail_item shortcodes. Remember that, the mega_menu_thumbnail_item shortcode will always go into the mega_menu_thumbnail shortcode. The mega_menu_thumbnail_item shortcode has the url attribute for inputting the URL of the menu item and the image_url attribute for inputting the thumbnail image URL. Take a look at the example below-

[mega_menu_thumbnail]
  [mega_menu_thumbnail_item url="http://example.com" image_url="http://example.com/images/image.jpg"]
    Menu Title Here
  [/mega_menu_thumbnail_item]
[/mega_menu_thumbnail]