My app needs to compatible with iPhone & iPad and based on iOS 6.0.
I want to know if i don't set spotlight icons or settings icons,what will be displayed? blank or use app icon instead?
Spotlight search icons are not required. Even if you don't provide them, your app icon will be used in the search results.
Using spotlight icons is recommended by apple in case you want to make sure they appear properly on all device types.
You can find additional info here.