There are several threads including scans of Crosby covers. Here are 2:http://goscf.com/t/21770http://goscf.com/t/40087
Crosby is best known for his cachets printed by thermograving which typically included a small real photo. He produced not only FDCs but naval covers and others.
Thermograving is a two-step process where wet ink is dusted with a powder that, when heated, expands to give a sort of imitation of real engraving but without its high initial cost. You have probably seen this on business cards.
Hopefully, you can get some idea of how thermograving looks. It is raised (like engraving), shiny (unlike most engraving), rounded (unlike most engraving) and is not as sharp as engravings.
Just because an older cover is thermograved doesn't automatically mean it's a Crosby cover. There are also covers with tiny photos that may or may not be by Crosby. The little Crosby photos are found detached, so don't believe faked "error" Crosby cachets with the wrong photo.
Thermograving can turn out badly. If the printing ink is applied too thickly, then the final work results in blobs; one more step so more spoilage. So we should appreciate the amount of work that went in to produce Crosby covers.