Simply double the resolution of your terrain.png. Liquids will no longer be animated, bricks and gold blocks will be a little buggy-looking from some angles, and fire from the firestarter will look weird, but you'll have your green lava.
Edit: Apparently there's a patcher that fixes the brick/gold/fire issue with high res texture packs, and you can still have animated water with your green lava. Just make sure Animated Lava is unchecked. viewtopic.php?f=25&t=46173