How can I restore the original wallpaper without formating the phone?
The standard wallpaper is in a:\System\data\euro.mbm . You can only access it using a 3rd party file manager (like FExplorer or SeleQ). In order to replace your current wallpaper with the default copy the a:\System\data\euro.mbm to c:\System\data\ and then rename it to backgroundimage.mbm (you’ll need to replace this file so if you have trouble just delete the backroundimage.mbm before renaming the euro.mbm).