The covers aren't always important.

PS had the train.
CoS had the Ford Anglia.
PoA had Buckbeak.

(All modes of transport! LOL)

GoF had the Horntail.
OotP had the Phoenix.

So, I wouldn't based any theories on the cover.

After all, DD being important/significant is no real shock -- after all he did defeat the first Dark Lord, Grindelwald.

The only cover that seems to give anything away is the adult and even that tidbit is tiny.