We are spending a considerable amount of money on creating fantastic illustrations that will accompany a learning guide. These illustrations are very unique and take 20-40 each to complete.

Once we go live with the site I know that a lot of people will be doing "Save Image As" and pass all of our hard work and money spent as their own. Are there any technical ways to protect these illustrations? 

Any JS solutions can by bypassed by disabling it in the browser settings. People can also take screenshots. Adding credit lines at the bottom of each images can be cropped out plus it will ruin the look and feel.

Are there any other things out there? or Is there a way to leverage the copied materials?

If the answer to those is no then is there a way to make it more difficult?

We want to use legal options only as last resort.