Like many others, I make extensive use of the art gallery. I grab pictures of people or creatures to show the players. I shrink down the monster pictures to make 1"=5' scale tokens. I hand out magic item pictures to keep things more exciting than just another +1 longsword.
Anyway, here are some of my favorite pieces.
My current favorite is from
Spell Compendium:
William O'Connor's "Divine Interdiction". Nice use of light in that piece, and it's unusual in that it shows the good guy (gal) losing.
Another nice Oconnor piece is
"Giant Size" from
Complete Arcane. It gives a good sense of scale, and the misty trees in the background are visually pleasing.
I like
Matt Cavotta's "Warmage" also from
Complete Arcane. It's a dynamic pose, and the guy looks half crazed, which is how I imagine warmages would be.
Speaking of Cavotta, I also like his
"Dimensional Sextant in Use" picture, which is not in the hardcopy of
Manual of the Planes, but is in the online art gallery.
Arnie Swenkel's "Planeshifter" from that same book is another fine piece.
Draconomicon is chock full of great art. I particularly enjoy
"Clinging Breath" by Ron Spencer -- take that, pesky adventurers!
"Dracolyte" by Steve Prescott is more of a strait-up portrait than an action scene, but very well done. And of course
Todd Lockwood's cover art is just fantastic, the kind of piece that makes me want to strap on my armor and go dragon-huntin'.