Over the past two years, inside the high-tech sanctuary of Industrial Light and Magic, the man who built a virtual virgin jungle for the last “Indiana Jones” movie and conjured 150-foot-tall aliens for “War of the Worlds” has been confronting his most difficult task yet: creating a digital version of the beloved Teenage Mutant Ninja Turtles that could realistically interact on screen with Megan Fox. On this assignment, Pablo Helman needed more than just turtle power. “Teenage Mutant Ninja Turtles,” the live-action reimagining of the 30-year-old comic book franchise in theaters Friday, features a completely computerized version of the four sewer-dwelling superheroes. The revitalized reptiles were fashioned at ILM by blending computer-generated imagery with several motion-capture performances by four actors. It’s a radical departure from the original ’90s film trilogy, when Jim Henson’s Creature Shop crafted puppety suits for actors playing the half-shell heroes.