Thanks for the feedback. Adding a custom thumbnail is a feature I would like to add at some point but is quite a bit of work, so I simply have not gotten around to it. Not all websites have an easily retrievable image in their html which causes some of them to not load anything. Also, try clicking the reload button that appears when hovering over a bookmark, as that will try and download a new thumbnail.

Edit - 5/30/2023: this feature will be coming hopefully within a couple months