@TheMongoose: If you can, run all your images through identify, the expected output should be something like what ixtab posted (the key parts being the resolution: 758x1024, and the format: PNG 8-bit 256c).
A simple batch job will probably be the quickest way to do that.
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Hopefully, @srm's issue will also be fixed in the next release, which should make the cover mode much more lenient with old/broken mobi files, and try harder at using *someting* as the cover. For those files, in might not always be the actual proper cover, but in most cases, it will be

.