As I supposed, this option is already included in Lubuntu for users: fortunately, no files of the system have to be modified and no Terminal command lines are necessary to do this editing.
I found the way by myself after some trial and error. Only then I saw that it is (obviously) also precisely described on the official Lubuntu manual page, here.
First, find a good icon image you like for your Lubuntu panel and
save it in your local drive.
Then, just go on the panel and right-click on it.
Click on Configure Panel.
Go to Widgets bar.
Click on Menu or Application Menu (it depends on your Lubuntu
version).
Here a new window appears, showing the path of the image file used
as the menu icon. If you like, you can back up the default path by
saving it on a .txt file or as a screenshot, for any problems may
occur or just to recover default settings.
Now just type the new path for the main menu icon, corresponding to the path of the image you previously saved.
For older Lubuntu versions, you can also simply right-click on the menu icon and the path of the image file used as icon is shown: you can now change it by entering the path of the new image you want to use.
As I supposed, this option is already included in Lubuntu for users: fortunately, no files of the system have to be modified and no Terminal command lines are necessary to do this editing.
I found the way by myself after some trial and error. Only then I saw that it is (obviously) also precisely described on the official Lubuntu manual page, here.
Then, just go on the panel and right-click on it.
Click on Configure Panel.
Go to Widgets bar.
Click on Menu or Application Menu (it depends on your Lubuntu version).
Here a new window appears, showing the path of the image file used as the menu icon. If you like, you can back up the default path by saving it on a .txt file or as a screenshot, for any problems may occur or just to recover default settings.
Now just type the new path for the main menu icon, corresponding to the path of the image you previously saved.
For older Lubuntu versions, you can also simply right-click on the menu icon and the path of the image file used as icon is shown: you can now change it by entering the path of the new image you want to use.