It's a digitally created semi-nude of Jessica Alba, not the real thing? She was actually wearing a sports bra type top and exercise shorts.. They just cgi'd it out and gave the "artist's impression".
But it is a fun film.
She was never fully nude in the movie anyway. The shot you speak of was a shower scene, shown from the side, with her arm strategically blocking the good bits. Surprisingly, Lindsay Lohan let the girls out in full view.