Mind the "OnlyShowIn" and AutostartConditio options, you might not want them

mirko@WizBox:~$ cat /etc/xdg/autostart/custom.desktop
[Desktop Entry]
Type=Application
Name=Files
Exec=/bin/xtest
OnlyShowIn=GNOME;Unity;
AutostartCondition=GSettings org.gnome.desktop.background show-desktop-icons
NoDisplay=true
mirko@WizBox:~$
mirko@WizBox:~$ cat /bin/xtest
#!/bin/sh

exec 1>/tmp/custom.log 2>&1
xrandr -s 1024x768