go to: wppizza->add new

add title, description, any preservatives the item might have (editable from wppizza->settings->additives), select pricetier (editable from wppizza->settings->meal sizes) and add prices as required.

make sure to also select at least one category.

if you wish to add an image, either set a featured image for this menu item (if available in your theme) or add an image as normal in your description.

the size of the featured image displayed will depend on your thumbnail size set in settings->media: thumbnail size if your theme does not allow featured images, and you are adding pictures directly into the description, you might want to turn off “display placeholder image” in wppizza->settings->layout

alternatively, add add_theme_support( 'post-thumbnails'); to your themes or child themes function.php file