-dPDFSETTINGS=/screen (screen-view-only quality, 72 dpi images)
-dPDFSETTINGS=/ebook (low quality, 150 dpi images)
-dPDFSETTINGS=/printer (high quality, 300 dpi images)
-dPDFSETTINGS=/prepress (high quality, color preserving, 300 dpi imgs)
-dPDFSETTINGS=/default (almost identical to /screen)

Autre lien
http://flukylogs.blogspot.fr/2012/08/gs-ghostscript-cheat-sheet.html