seitime-frappe/frappe/docs/user/guides/app_development/custom-module-icon.md

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