Board Thread:Support Requests - Getting Started/@comment-28089029-20160612125943/@comment-5811199-20161207033614

You can upload File:Placeholder_person.png to your wiki - and if an image doesn't exist, it will put the placeholder image.

Alternatively, you can do what Wolfblood Wiki did - they added a magic word to the category: Category:Pages with broken file links - this prevents the category from being seen on the page, and on any other pages with broken file links.