This from the US Copyright Office website regarding Fair Use:
"Section 107 contains a list of the various purposes for which the reproduction of a particular work may be considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research. Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair.
1. The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
2. The nature of the copyrighted work
3. The amount and substantiality of the portion used in relation to the copyrighted work as a whole
4. The effect of the use upon the potential market for, or value of, the copyrighted work "
In the case of covers in a collection available at DCM, they would be for "nonprofit educational purposes," the cover being only a small portion of the complete copyrighted work, and would be considered for teaching, scholarship and research. As to the effect on the potential market, DC and Marvel have done book collections of covers in the past but it is unlikely that other publishers would.
Perhaps this is something that would have to be decided on an individual project basis.