I think there's a glitch with the postbit cover images at the moment, when I upload one myself it stretches it to extremely large proportions, even one as small as 50x50, then when I change the options and select "auto" to make it look normal, I get an error because I need to re-upload the same image again, which puts me back to square one. So I would say it's probably best to not worry about postbit covers for now, and hopefully @Jay can get in touch with the author of the add-on to see why this error exists (didn't happen before) and if there's a solution. The way it worked before, practically any sized image would work and it would scale to the size of the postbit quite nicely. Now it looks...well, like it does in my postbit.
Edit -- ...and now for some reason it's working...typical, lol