The Schema plugin has a built-in extension called Default Image, it works like a fallback to cover missing Featured Images on your site content.
Google requires image in Article, NewsArticle, and BlogPosting markup.
Every page must contain at least one image (whether or not you include markup). Google will pick the best image to display in Search results based on the aspect ratio and resolution.Source: Google
Solve Markup Errors
This Default Image extension can solve several errors in Google Structured Data Testing Tool which is caused by missing images -for example- in Article or BlogPosting markups.
The extension allows you to specify a default image to be used on those Articles missing Featured Image.
Default Image settings
To enable this feature, go to Schema → Settings → Schemas → General → Default image
Use this setting to upload an image to be a fallback for missing Featured Images.
Don’t forget to use an image that can present your overall content.