I just recently discovered this website with all its amazing classes and resources so I haven't taken Lee's class on business but I did invest in this book when I started freelancing: http://www.amazon.com/Business-Legal-Forms-Illustrators-Crawford/dp/1581153643. It covers a lot of contract situations (anywhere from book publishing to collaborative projects to lectures) and has really helped me navigate and prepare contracts for sticky situations.
With that in mind, I would agree only to allow them to use the work for a specified period of time within a specific set of boundaries but that the rights of the images remain yours after the termination or end of the agreed time period/license. If they wanted to buy the copyright to your work you can also offer that for an additional (high) fee. You didn't mention it before but with work for hire contracts royalties aren't always a given. I would look into that as well.
I agree with @Suzy-Heitz that you should definitely see if they are willing to negotiate and work with you on this.
Keep us posted on what happens! Best of luck.