How do you change/add box arts to games?

[nordlead]

@Grampy, That is a completely different issue. Soriku was asking about adding boxarts to games already in the game database.

you are refering to adding games from the game database into your game collection. To do that, to to Games->Game Database on the top bar. Search for the game you want and click it. Then click "Add to my game collection" under the Game Owners tab.