You just have to read this and please do yourself a favor and read it all: How to find a cover image for a book with an ISBN
In case you've ignored my above advice a short summary: "isbndb.com does not have the functionality you want.". The site recommends you to use Open Library Covers API
], which is quite simple to use.
ISBNdb.com doesn't actually provide the cover image. They have an useful API for finding out more information about a given ISBN, but not the cover image.