Wrong resolution after installing LXDE

FfReAk

Hello,

I have a raspberry pi 3 with retropie 3.7 installed. Yesterday I installed lxde via the retropie setup script and during this installation I was asked if I would like to keep my existing settings or if I want to use the new ones coming with the installation of lxde (I don't know the words exactly, but my options for answering were "Y/I = new ones" "n = old ones" or something like this). I chose the new ones. After the installation, the resolution of emulationstation was very low and it looked blurry. If I quit emulationstation, the resolution of the commands are very low, as well.

The weird thing is that I had a backup of my whole sd-card (a bin-file) before the installation of lxde. I wrote it back on the sd-card and started the raspberry, but the resolution was low again. At the time, I made the backup, I surely had the right resolution.

My questions are:

How can I fix the resolution back to normal?
How could that happen that despite of backing up my whole sd-card, after writing it back to the card, the resolution still keeps being wrong? Does the raspberry have an apart memory (a firmware or something like this) where the resolution is saved independently from the sd-card?