"Jesus and Jonah" by J.W.McGarvey

Thanks a lot! Your effort to fill the glaring gap of RM resources in Logos is greatly appreciated!

I know that pictures can be or are copyrighted. (...).  Are book covers like this considered copyrighted material? 

This might depend on the jurisdiction, but in many cases they will be - which in effect is the reason why we even get some Logos resources without book covers. The book cover would often be considered copyrightable as artwork - the so-called "threshold of originality" lies very low in some countries - only the book title and author name in a common type on a single color background (like they often did bookcovers in former times - or like the book covers of printed books beneath the so called "dust jacket") would probably not suffice, but once pictures or photographs are contained, and someone "designed" it, it is copyrightable.  

Original book covers, even when above the "treshold", of course could be old enough to fall into the Public Domain like the book themselves (but even then there's legal discussion whether the reproduction of a twodimensional, PD artwork creates a new copyright for the individual who performs the scan or takes a picture!), so original covers taken from archive.org or such sites probably are safe to use in most cases, but again even this could be an issue. A modern cover used by a recent (re-)publisher for an old book would probably be copyrighted.

How do we know if we can use them?

I think that using such a cover for a Personal Book is legal in most jurisdictions (you in the US are probably easy off the hook with the "fair use" exception to copyright) - and I use it for my version of the PB as well. After all, this is private and educational use.

Posting it here in the forum may be another matter altogether. Still another matter is the question if someone takes offence - and goes after the respective users or the company hosting the platform (e.g. in Germany there is a veritable 'industry' of unscrupulous lawyers living off cease-and-desist-letters for such copyright matters). More prudent might be to use original covers - if available - and say "there's also a nice modern cover to be found on Google image search" or something like that.  

My 2c from the outer rim of Logos world

Thanks a lot! Your effort to fill the glaring gap of RM resources in Logos is greatly appreciated!

This expresses my thoughts exactly! Thanks!