Custom thumbnail from URL or local storage

[cantunborn]

Is your feature request related to a problem? Please describe.
Sometimes thumbnails are not detected due to absence of meta image tags or other reasons. In this case there is no apparent way to manually add an image as a thumbnail (remote or local).

Describe the solution you'd like
Capture a screenshot of the site if image finding condition is not met (issue open already), or let the user add custom image from a URL or local storage.

Describe alternatives you've considered
No alternatives since UI does not provide any way of editing thumbnails.

Additional context
A similar feature is present in raindrop.io, where it takes a screenshot as a fallback but also lets the user add custom thumbail from a URL.