First of all, sorry for starting a new question. I couldn't see the forest thrue all the trees anymore is what we would say here in holland.
It started with this question where I didn't know how to switch to my optimal screen resolution. Sylvain Pineau helped me out with the solution to use the xrandr code in the terminal. That worked out, but i fail to make it go to that resolution (1440x900) when my computer start. Even the redirection to this question didn't help me out. The fourth answer on that page did look similar to what I had done with the xrandr code. But when I have made the changes to lightdm.conf and want to save it, it says that it is a read only file wich I can't overwrite.
Does anybody know how to fix this? Thanks in advance for your help!
I used this answer, becouse it seemed the most simple solution. I just don't understand the script wich is used in the terminal yet. But this answer below said that I needed to change the lightdm.conf
I've had a similar problem with my laptop as above, and I wanted to add something to the conversation. If you don't care about the login resolution (I don't), but want the actual workspace to be the right resolution, you can add your monitor information to LightDM but not turn on full resolution until you login.
I just added the following script for my monitor in a file: lighdmxrandr
xrandr --newmode "1440x900_60.00" 106.50 1440 1528 1672 1904 900 903 909 934 -hsync +vsync xrandr --addmode VGA1 1440x900_60.00
then I call it in /etc/lightdm/lightdm.conf
Once I login I can setup my monitor in Displays and it works, even after I re-log (used to give me a big ugly error).