Check there:
http://airliners-atc.com/forum/member.php?u=1
There is no flag there, just ISO countrycode.
And in UserCP when choosing my flag only ISO country code shows up, not a flag.
I have changed the img src path to full address path like:
<if condition="$post[fieldXX]"><img
src="http://airliners-atc.com/forum/images/flags/$post[fieldXX].png"
Is fieldXX wrong?