It just might be to differentiate from the good and evil Garland. We all know he use to be a knight of Coneria and then he turned on them. We've seen this type of thing before when a character changes sides and changes his appearance. i.e. CecilOriginally posted by Black Mage 121
It shows both good and evil. Nothing more. If the character in the opening FMV was Garland, why not use the Garland depicted in that piece of artwork? Why create a 'second' Garland?