Peter Markowski, who works for Warner Brothers Animation, recently posted some great conceptual art images from the "Homecoming" episode of Green Lantern: The Animated Series on his blog. The seven images he released focus primarily on vehicles and the Maelstrom.
Another artist, Zak Plucinksi, posted a couple of character design images for Ch'P and Tomar-Re on his blog as well, both of whom will appear in the next set of episodes. Interestingly he was designing Bd'g, Ch'P's successor, and was going for the more rough and tumble look for the character, but the decision was made to go with a less imposing design to play off of the unassuming nature of the character. He has also posted a great deal of design images for characters we've already seen. While all of the images can be found here, it's worth visiting their sites to see all the hard work that these artists do to bring the animated series to life.
Sources: Peter Markowski, Zak Plucinski Art Blog