According to this news release, it appears the first test firing of a Hellfire missile from a Predator took place in Feb of 2001, a few months after the (probable) bin Laden images were taken. However, there were other military options available, including an air strike or perhaps an attack using the SLAM-ER missile, which can be retargeted in flight.

Let's not forget that Sudan offered to hand bin Laden over to the US but the Clinton administration declined the offer for legalistic reasons. That's what happens when you have a cabinet made up of a bunch of lawyers. You can hear Clinton explaining his decision here.