Ok guys, problem solved!! Thanks to all of you for helping! This is how i solved the problem:
1) With Synaptic, install the package named ARANDR.
2) Then, open ARANDR
and set the resolution to the one you want as "default" (1360 x 768 in my case), and press the "apply" button.
3) After the new resolution is applied, press the "save as" button.
4) By default, ARANDR will ask you to save this monitor settings in this folder: /home/YOURNAME/.screenlayout
... do it (lets say you name the file "defaultresolution").
5) With the File Manager, go to the location of the newly created file (/home/YOURNAME/.screenlayout), right click on the file defaultresolution.sh
and make the file executable (by checking the box).
6) Then, go to this location: /etc/xdg/lxsession/Mint. Inside this folder you will find a file named autostart
. Edit the file, by inserting the following line: @/home/YOURNAME/.screenlayout/defaultresolution.sh
. (what you just did, is order your lxde to "autorun" this command at startup).
7) Save the changes, and restart your computer. If everything went well, your lxde mint will show the desired resolution as default.
PS: special thanks to user seppalta