The original Montenegro flag icon is 16x12 instead of 16x11 for some reason, so in the current sheet, it's been resized to 15x11. This makes it off-center. It looks better as a "squashed" 16x11, so I took the liberty of adding that to the sheet (it's the one with the gold border halfway down the second column).

If you're particularly attached to having it be 15x11, then the .flag-me class needs a width:15px added to it.