Unlock the following Art Galleries by beating the corresponding games:
ART GALLERY - GAME Art Gallery 1 - Metal Slug Art Gallery 2 - Metal Slug 2 Art Gallery 3 - Metal Slug X Art Gallery 4 - Metal Slug 3 Art Gallery 5 - Metal Slug 4 Art Gallery 6 - Metal Slug 5 Art Gallery 7 - Metal Slug 6 Tonko Gallery - Beat all seven Metal Slug games with limited continues.
To get easy tokens, after you defeat the Final Boss on Metal Slug 3 or Metal Slug 5, save the game and quit to the main menu once the credits start. You will get five tokens. Reload the game and quit to get five more tokens. Repeat as desired.