I don't think it's a problem any more than it's a problem for the people packaging the Fleischer Superman cartoons to use screenshots on the DVD cases. I think there's a specific law that protects that.
It may even be easier than that. Unlike Superman, whose appearance you can argue is a trademark of DC's (unregistered), it's not like Buck has some iconic appearance. Even the classic iconic Pulp cover is of some other guy...
That said, a lot of the fringe rules like this are more internal relating to being a good citizen when it comes to trademarks or avoiding companies that have proven to be obnoxious about their rights and less about the legal footing, so it depends on where we consider that, too.
(I should mention that Fair Use is, unfortunately, something that comes up as a legal defense. In theory, that sounds fine, but if we're in months of litigation, it's still expensive to win that victory. That's why so many things get licensed when it's not necessary.)