How can I rotate my display using some GUI tool, in LXDE?

You can use arandr, a GUI front end for xrandr. This also is capable of rotating the screen. It is in the Debian repositories.

Arandr's web page also mentions alternative GUI tools, no idea how up to date that is.

The good tool for this job is xrandr.

First determine your display with something like

$ xrandr --verbose | awk '/ connected/{print $1}'

Now you are ready to play with it

xrandr --output LVDS1 --rotate left
xrandr --output LVDS1 --rotate inverted
xrandr --output LVDS1 --reflect x
xrandr --output LVDS1 --reflect xy

and when you mess your screen too much just run

xrandr --output LVDS1 --rotate normal
xrandr --output LVDS1 --reflect normal