You can create a button and assign "Insert from URL" to it with:
"http://covers.librarything.com/devkey/KEY/medium/isbn/" & Tablename::ISBN
in "Specify URL".
It is also possible to use this step in a script with the exact same parameter and then assign that script to a button
Your input appreciated, took a few times to get it right with a single space after & and the API offered by librarything was not nearly as good as I thought it would be...it returns a transparent 1×1 pixel GIF image approx half the time for my library collection, this was really throwing me. Switched to http://bigbooksearch.com/books/. Result works well, here's a screengrab of current solution