You'll need to use fs for that: http://nodejs.org/api/fs.html
And in particular the fs.rename() function:
var fs = require('fs');
fs.rename('/path/to/Afghanistan.png', '/path/to/AF.png', function(err) {
if ( err ) console.log('ERROR: ' + err);
});
Put that in a loop over your freshly-read JSON object's keys and values, and you've got a batch renaming script.
fs.readFile('/path/to/countries.json', function(error, data) {
if (error) {
console.log(error);
return;
}
var obj = JSON.parse(data);
for(var p in obj) {
fs.rename('/path/to/' + obj[p] + '.png', '/path/to/' + p + '.png', function(err) {
if ( err ) console.log('ERROR: ' + err);
});
}
});
(This assumes here that your .json file is trustworthy and that it's safe to use its keys and values directly in filenames. If that's not the case, be sure to escape those properly!)