For that, you can go on this quiz. Their you've got all the flags saved on jetpunk database. On the flag that you want, you can right click and copy the URL of the image.
Then, in your hint cell, you copy this : img src="URL OF THE IMAGE" width="20" height="15". You add < at the begining (before img src) and > at the end (after height="15"). Then, you replace the URL OF THE IMAGE by the url of the flag you've copied.
I don't know if it's clear. If it's not, don't hesitate to ask :)
Then, in your hint cell, you copy this : img src="URL OF THE IMAGE" width="20" height="15". You add < at the begining (before img src) and > at the end (after height="15"). Then, you replace the URL OF THE IMAGE by the url of the flag you've copied.
I don't know if it's clear. If it's not, don't hesitate to ask :)