New screen some issues

I finally got my dell 34" ultrawide yesterday, i unplugged my old main monitor and plugged in the new one, i got 2 screens coected. It all works as it supposed to except i cant seem to be able to change the wallpaper for some freakin reason, its like the system still thinks my old main screen is coected or something...

heres my xrandr output if that might be to any help: http://pastebin.com/UW9utsHT

And while im at it, how do i overclock my new monitor in linux? works fine at 80hz in windows. i cant add custom resolutions with xrandr to the screen i got problem with it says
xrandr --verbose --addmode DP-4.8 "3440x1440_80"

X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 140 (RANDR)
Minor opcode of failed request: 18 (RRAddOutputMode)
Serial number of failed request: 49
Current serial number in output stream: 50

this is what i used to add the new mode xrandr --newmode "3440x1440_80" 571.75 3440 3712 4088 4736 1440 1443 1453 1510 -hsync +vsync

might add that im using xubuntu if that makes any difference