^Actually each character has different kind of fruit and choice of fruit is connected to their story in TVD.
Katherine has pomegranate - fatal fruit from story of Persephone, where it has dual meaning of life and death, rebirth and eternal life. Also she is femme fatal.
Klaus has a pear, sign of eternal life. I'm not sure what Stefan is holding - an orange or maybe an apricot. Damon is holding an apple, forbidden fruit. His feelings for Elena are can be seen as forbidden fruit. Also he has to choose between being more human or being more vampire.
I'm really surprised designers and especially producers went with something like this. It's complex and certainly not obvious. I like that posters look like paintings.