$ sudo apt-get install libgd-tools

Create image in Gimp

Create a suitable image in Gimp, ensuring that it has a transparent rather than a white background.

From the menu - Image → Mode → Indexed and “Generate Optimum Palette” with “Maximum number of colours: 255”.

Save result as a PNG file, e.g. “router.png”

Convert to GD2

$ pngtogd2 router.png router.gd2 0 1

$ sudo -i
# apt-get install nagios3
# cd /etc/nagios3
# htpasswd -c htpasswd.users nagiosadmin

Then point your browser at http://localhost/nagios3/.