if you're referring to the larger "print-style" covers i think those are restricted to premium comics and novels, at least as the series image; but you can still have one as the first episode of your series or posting it with the first page(s)
if you're looking for pointers on what makes for an appealing thumbnail or banner there's a few threads spread about here on the forums but i also recommend giving this Medium article by Tapas a read
hope it helps and well wishes on you new series!~