That's not necessarily the case, actually. Aussie has spoken with a couple of copyright holders to allow their work to appear here (with disclaimer and credit), though I can't think of any examples. And also, though Aussie usually refuses a book from a series with even one copyright, renewal or a lack of renewal on one issue doesn't necessarily have anything to do with the renewals for any other issue in the series.
The copyright on an issue also might not cover the copyright for the specific image, if it was copyrighted separately.
That all said, I'm fairly sure (for reasons deeply buried in the previous paragraph...) that the entire series is public domain and I seriously doubt Eastern Color copyrighted any of the covers.
(As a backup, check with your company's lawyer and see if he thinks it would qualify as Fair Use. In a book about comic history, you should be able to plausibly use any image you like, as long as it's not too many or too complete a duplicate.)