Selling custom LEGO image mosaics using original LEGO bricks without breaking LEGO's Fair-Use Policy
First of all, if you are serious about this business, go to an IP lawyer who can take this up for you and give you a much more confident answer than the bunch of us LEGO fans can :)

LEGO's Fair Use Policy is located here.

Secondly, you might want to read the perspective in this thread on Bricksetforum. Lots of interesting points of view, and, decent information about what you should be able to show, and what LEGO's claimed and likely actual trademark rights are.

Finally, did you talk with that IP lawyer yet as per my first point? hint hint