I think the covers have more to do with the story than the title...
for Twilight - the apple represented the fobidden fruit (like the garden of eden, that's why there's the quote from Genesis in the front)
for New Moon - Stephenie said she has no idea why the publishers chose the tulip, but she liked it, lol
for Eclipse - I don't know Stephenie's ever explained it, but to me (and judging from the poem in the front) the fire (Jacob) and the ice (Edward) are pulling the ribbon (Bella) apart till she breaks... just my opinion
for Breaking Dawn - Stephenie explained that the cover meant two things... first, it represented Bella's position in the vampire world - moving from the weakest player (the pawn) to the strongest (the queen) ... second, it foreshadowed that the end battle would not be a bloody massacre but a strategic play for power like a chess game.
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as for Midnight Sun... i personally think the cover will have more to do with something pertaining to Edward and the story rather than a sun exactly (the rest of the book titles had references to celestial elements, but there were no suns or moons or anything like that for them) ... but who knows, I could be wrong, lol...