If you want to create a custom icon for your module, you will have to create an SVG file for your module and set the path to this file in the desktop/config.py of your app.

This icon is loaded via AJAX first time, then it will be rendered.

Example:

from frappe import _

def get_data():
    return {
        "Frappe Apps": {
            "color": "orange",
            "icon": "assets/frappe/images/frappe.svg",
            "label": _("Frappe.io Portal"),
            "type": "module"
        }
    }

> PS: A great place to buy SVG icons for a low cost is the awesome Noun Project