*Just put SkyrimPrefs.ini to "My Documents > My Games > Skyrim" directory.

*Set iScreenShotIndex 0 to [Number of your Screenshots] + 1 if you don't want to overwrite your screenshots.

*Find:
iSize H=600
iSize W=800
lines and change it with your screen resolution.

*For more shadows:
iShadowMapResolutionSecondary=0 to 1024 or 2048 or 4096
iShadowMapResolutionPrimary=0 to 1024 or 2048 or 4096

*Backup your old SkyrimPrefs.ini, just in case.