In order to add a flag for your country, you need to create a matching locale for your country. You can do this even if you don't translate anything into a new language. Here's how to create a new locale:

As admin, go to 'System' then click on the 'Regional' tab. At the bottom of the page, you will see a 'Create new locale' button. Clicking this will take you to the Locale Form.

Once you enter all the information and save the locale, you will be able to use it for your blogs and your posts, which will then show the flag for the new locale's country.