Main Menu

Let's say you want to use your module icon as your main menu icon.

Make sure this file exists at:

module_name/static/description/icon.png

In the XML where you define your main menu, add the web_icon attribute:

<menuitem name="My Application" id="module_name.menu_root" web_icon="module_name,static/description/icon.png"/>

Then upgrade your module.

Application Menu

To set the application menu, just add your icon.png inside your description folder:

module_name/static/description/icon.png

Then restart odoo-server