Default Thumbnail for Magazine content element

I have the same question as this thread but had trouble implementing the suggested code.
https://kriesi.at/support/topic/change-the-default-blog-thumb-icon-with-a-default-image/
On my Home page I’m using the Magazine content element. For posts that don’t have an image, the default thumb is the pencil icon. I’d rather use my own default image. How can I do this without having to use the WP “featured image” option?

When I added suggested code the pencil disappeared but only the background color remained. Do I need to resize the image I want to use to 49×49?

Thanks
Mike