Thank you kiwidude. Unfortunately I don't a website, but here is what I can share as I just created the problem and the solution. From Feedbooks.com I grapped the ePub The Bandit of Hell's Bend by Edgar Rice Burroughs.
I imported it into Calibre, and send it to Aldiko by way of Calibre Companion and imported it into Aldiko. Cover thumbnail has a white border on the right hand side.
Deleted it, then went back to Calibre and converted it. Send it back to Aldiko via the same process, and no white border on the right hand side.
I had the same result for several books from Feedbooks.com in this process. I've also seen this from other ePub sites.
As far as Calibre goes, I know of no setting in it that I see turned on where it solves this problem. But if I can avoid a conversion for every ePub that seems a good route.
Thank you for your assistance.