This is rather complex on Windows Phone 8.
Regarding the splash screen (if any), you can use special file names:
- SplashScreenImage.Screen-WVGA.jpg (for WVGA phones)
- SplashScreenImage.Screen-WXGA.jpg (for WXGA phones)
- SplashScreenImage.Screen-720p.jpg (for 720p phones)
For all other images, you have to implement an own class that returns the best image for the current resolution. This is explained in the following MSDN article: Multi-resolution apps for Windows Phone 8.