Metal Slug Anthology Codes -

Desktops 1:
Successfully complete all seven Metal Slug games under the Normal difficulty setting and limited continues.

Desktops 2 and Special Music:
Successfully complete all seven Metal Slug games under the Hard difficulty setting and limited continues.

Art galleries:
Successfully complete the indicated game to unlock the corresponding gallery.
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: All seven Metal Slug games under the Easy difficulty setting and limited continues.