If you want to increase your earnings you have to find a system that works for you. To built a system you need to spent a lot of time in knowning how the game works you want to earn money on.
If you want to find wallpapers about the free category or just nice free wallpapers for your computer you can find them at many sites. What you want to do is find one that matches your feeling at that moment.
It is very easy to change your wallpaper so it is a good idea to keep a folder on your computer where you can store all your downloaded wallpapers. Maybe you see it as free desktop wallpaper but is very good to do this.