ULTIMATE MARVEL VS CAPCOM 3

To unlock and get all the items in the gallery you will have to end the arcade mode with all the characters!

And as Tomjeh reported you have to also complete one mission (combo training) with each character.

This will also give you the following achievement:

If you want to do this the easy way, set the difficult to Very Easy. On the character select menu, choose your two best fighters and a character that is still required to finish Arcade Mode in order to see their endings.

On Very Easy you won't need to change your first character, because it is such an easy mode! When Galactus appears, finish his minions and waste 3/4s of his energy with your main players. Now switch to the third character, and spam their super moves when able. Do this with all the characters and the achievements are yours!

Also note that when you are done with all the characters , you will have to play one more time and when you face Galactus let him beat you, in order to see his ending too .

If you had downloaded Shuma Gorath and Jill DLC, then you will have to see their endings too in order to unlock their collectibles.

Thanks and good luck :)

Just as an FYI, while the DLC characters (Jill and Shuma Gorath) do not count towards the "view all endings in arcade mode" achievement, they do count if you own them for this achievement. When you scroll to gallery, in the top right corner you will see a percentage, and without beating arcade mode with Jill and Shuma Gorath to unlock their endings and artwork, you will not be able to reach 100%.

Play through everyone's Arcade Mode story to get every Gallery item.

NOTE: Hit RB on the main menu of the game to see your License. Here you can track your progress.