just use the “featured image” for a menu item
if you do not have this options it’s either hidden (go to “screen options” at the top of the page to enable/show it) or your theme does not have thumbnails enabled (tell the author about it. not my fault)

if the author doesn’t want to or cannot do anything about it, you can also try to just put the following code at the bottom of your themes function file (before the closing ?> if any )

add_theme_support('post-thumbnails');

if you do not want to display any images, delete any featured images that may be associated with your menu items and uncheck “Display placeholder image when no image associated with meal item:” in wppizza->layout