I will no longer provide screenshots here as other sites have far better in both quality and quantity. I'll link to those sites instead for your benefit. :)
Great dvd quality, hundreds per episodes and specials, and has panshots - TV quality | DVD | Search database
Nice quality w/ watermark, hundreds per episodes and specials, and has ability to search for specific characters/places

Violent Red
Great dvd quality, hundreds/thousand per episodes and specials

Ok, I'll still host these small and kinda grainy screenshots of the original Avatar trailer, 'cause the other sites don't have the shots of Aang opening his eyes dramatically in Avatar State mode from the pilot. =/ *shrugs*

Sneak Peek Trailer