If you are creating new Recipe schema.org type to markup your recipes, you will need to upload at least one photo image of your recipe to be used in the markup output.
Schema Premium plugin has a default setup for the schema properties.
Upload a Featured Image
The Property > Image field source is set to: Featured Image
The image is pulled from WordPress Featured Image, so if there is no Featured Image attached, image will be missing from schema markup output.
To upload an image, you will simply need to upload a Featured Image for your Recipe page.
Adding Multiple Recipe Photos
To add markup to multiple photos of your Recipe, you will need to change Property > Image source to: New Custom Field, then click on the update button to save the new settings.
Now, when you Edit your Recipe page, you will notice a new field for Image under the Recipe tab.
Notice the Add Image button, this will allow create multiple photos in your Recipe structured data markup as you wish.