Lazarus_03: I do like in-game achievements, but I disagree with this; you shouldn't be forced to be a completionist to get more privilege on something you already own.
How about being able to choose from the wallpapers available in the 'goodies' section?
^Also this.
Alexim: I'm not sure I've understood it, but my suggestion is not to force anyone's hand, it's simply a way to make achievements more attractive as many people find them useless.
In this way there would be a small prize for those who take the trouble to complete them, while those who are not interested would still have available the basic background of the game.
Not everyone can complete all achievements - some are very grindy, others beat that on hard mode, etc.; it consumes time and if you're the type who beat a game then move on to next one, you'll miss this one as well.
So you're force to invest more time and/or keep trying, otherwise you'd just sit with one background per game despite owning the same thing as others.
As you said, not everyone is into achievements; but one might still be interested in that
certain wallpaper regardless.
They shouldn't need to go through all that to get those.
Alexim: More than anything I proposed it because many games have more than one very nice artwork and
would add variety of choice.
Agreed, so why not make them readily available for everyone?