Looks like I was wrong - or perhaps the person is taking a long leave of absence?! But I did find another link which appears to provide a fairly high-quality PDF front page. The format is
http://online.wsj.com/public/resourc...1-YYYYMMDD.pdf
So, for example, today's cover is
http://online.wsj.com/public/resourc...1-20180917.pdf
I'll bet this file is substantially bigger than the old, and (unfortunately) the name needs updating each day, but at least it is a much sharper image than the old jpeg. Would you still be able to fit it into the ebook?
For your reference, the old URL is
https://vir.wsj.net/fp/cdn/fp/assets/images/WSJ_A1.jpg
As you'll see, it's stuck on the 9/5/2018 edition. Thanks.