VGFacts

Currently, Super Smash Bros. Brawl is only game of the series to have unlockable characters on the box art. (This includes the JP and NA boxes.)
Starting characters of the 3 games for reference:
N64: Mario, DK, Link, Samus, Fox, Kirby, Yoshi, and Pikachu.

Melee: Bowser, Capt. Falcon, DK, Fox, Ice Climbers, Kirby, Link, Mario, Ness, Pikachu, Peach, Samus, Yoshi, and Zelda/Shiek.

Brawl: Bowser, DK, Diddy, Fox, Ice Climbers, Ike, King Dedede, Kirby, Link, Lucas, Mario, Meta Knight, Olimar, Peach, Pikachu, Pit, Pokemon Trainer, Samus/Zero Suit, Wario, Yoshi, and Zelda/Shiek.

N64 Boxes:

JP:
http://oi39.tinypic.com/14ty0ww.jpg
http://i.neoseeker.com/boxshots/R2FtZXMv...Et3gfy.jpg

NA:
http://wii.mmgn.com/lib/Images/News/norm...sole-1.jpg
http://gamesdbase.com/Media/SYSTEM/Ninte...ntendo.jpg

EU:
http://static3.wikia.nocookie.net/__cb20...rt_PAL.jpg

Melee Boxes:

JP:
http://i.neoseeker.com/boxshots/R2FtZXMv...9E4LGH.jpg

http://i.neoseeker.com/boxshots/R2FtZXMv...qYgzJG.jpg

NA:
http://i.neoseeker.com/boxshots/R2FtZXMv...ly8D7L.jpg

http://i.neoseeker.com/boxshots/R2FtZXMv...UtPAva.jpg

Brawl Boxes:

JP:
http://static3.wikia.nocookie.net/__cb20...xart_J.jpg
http://image.gamespotcdn.net/gamespot/im...6_back.jpg
NA:
http://admintell.napco.com/ee/images/upl...wl_box.jpg
http://image.gamespotcdn.net/gamespot/im...6_back.jpg

(Linked to boxes as the pictures are huge.)