Add image thumbnail of resource

[azaroth42]

Image thumbnail of Resource

(Provide link to index.md of the issue, if available. To be filled in after issue is created - you need the issue number!)

Use case

I want to be able to display a thumbnail image for a resource, be that a canvas, a range, an audio file or a video, such that it can be clicked to activate that particular resource.

[triplingual]

Does it matter whether this is declarative or via a service? Or should the recipe show both?

[kirschbombe]

I vote for showing both :)

[glenrobson]

We should change this one to be a manifest thumbnail and create new issue for canvas thumbnail.