I've never read the whole manga (tried to, but got bored very soon, to be honest), so I don't know if it's purely symbolic imagery or it features somehow in the plot.
Even if it's the second, then I don't see why it can't be both.
I don't really view the OP as being particularily separate from the main plot of the anime, so the inclusion of various allusions in the OP is only a sneak peak into the overall symbolism of the entire show.
It's not like the OP is telling a separate story. It's not a music video based on the anime, it's part of the anime, thus the imagery offered to us in the OP is part of the anime director's message.
The apple, the allusion to Michelangelo's Sistine Chapel
fresco (with Light and Ryuk instead of Adam and God), the contrast between the extremes of L and Light, the allusion to Sandro Botticelli's
"Birth of Venus" with Light as Venus. It's all part of the symbolism that DEATH NOTE is chanelling at the viewer. They are basic messages that tell the viewer something about the authors intensions for this show, what he wants us to see the characters as, etc.