The Art Gallery is a feature found in the 'Extras' menu in Skullgirls Encore. It shows a range of both guest art and official art, ranging from action shots to model sheets. It also includeds all cinematics found in the game.
The Art Gallery was added via a patch update on the Steam version of the game on November 1st 2013, with both console versions intending to receive it alongside the release of the Robo-Fortune patch update. Both the Xbox 360 and PlayStation 3 versions where supposed to receive the Art Gallery with the release of Skullgirls Encore, but due to several bugs amongst other difficulties it was delayed (with the situation regarding it's implementation remaining ongoing).
Art can be unlocked by achieving certain tasks in the game; beating Story mode with a character unlocks their individual action shot and movie-esque poster. Beating Arcade mode with that character unlocks that characters Line-up art, model sheets and concept art. Alternatively the player is also able to unlock all art pieces by beating Marie 300%.
To unlock everything (excluding two cinematics) you must beat Story Mode and Arcade mode with every character (including DLC). These two cinematics are impossible to unlock unless you beat Marie 300% - them being the cinematic for before and after beating Marie 300% respectively.
The Art Gallery also houses the Music Player, which can be accessed by pressing Start (or the Enter key if using a keyboard) while inside the Art Gallery. All in-game music is available to be listened here, including both the English and Japanese additions of In a Moment's Time (Hitomi No Kioku).
All of Valentine's official artwork and cutscenes were changed on May 13th 2014 via a patch update so any red crosses on a white background appeared fuchsia colored instead. This was to avoid any potential copyright claims regarding similarity to the American Red Cross's emblem.
Christmas and Ileum, who appear in various artwork and cutscenes, where also changed in a similar fashion.
Prior to the September 30th 2014 patch update, Fukua was the only character that didn't unlock her own set of artwork in the gallery. Instead beating story/arcade mode with her would have unlocked some of Filia's artwork.