Please give us a description of the issue

When I use the filter wpseo_schema_article and use the example code provided here, I see fatal error.

Steps to replicate:

1. I used the code snippet plugin to add the example code.
2. Changed the value of $attachment_id in the example. I added an image and used the ID for that image
3. Then I saved the code and activated it.
4. I checked a post where I set schema type as webpage and page type as article. I noticed fatal error in the page source that says: WPSEO_Schema_Image is not found.

Screenshot:

• I've read and understood the contribution guidelines.
• I've searched for any related issues and avoided creating a duplicate issue.