I want to add picture icon "new" when we update a new topic, how do you do it?

It looks like that section is a module. I think you can try this way:

Create a custom field for article so that you can set an article is 'NEW' or not
In layout code of this module, you will check if any article has 'NEW' custom, then load that icon on frontend.
Of course, you have to style for that new icon in module.

Regards