Terminal command notify-send

A limited set of HTML tags can be used inside
the notification body:

<b></b>
<i></i>
<u></u>
<a></a>
<img></img>

The notify-send utility provides an option we
can use to specify the notification icon. The
option is -i or —icon; it takes the path of the
icon we want to use as argument.
Which usually can be found as /usr/share/icons/

https://specifications.freedesktop.org/notification-spec/1.2/

Directory where icons live: /usr/share/icons/