il faut rajouter une résolution dans ton fichier xorg.conf
dans le terminal tu tape :
sudo gedit /etc/X11/xorg.conf
ta un fichier texte qui s´ouvre.
tu descend au niveau de ta carte graphique.
t´auras des trucs entre guillemets indiquant la réso :
Section "Screen"
Identifier "Default Screen"
Device "NVIDIA Corporation NV44 [GeForce 6200 TurboCache]"
Monitor "Vision Master" #"LS902U"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
tu rajoute la réso que tu eux si elle est manquante entre guillement pour tous les modes de couleur.