gistfile1.js

var css = 'insert lots of css here';

var files = {};
css = css.replace(/url\(["']?(\S*)\.(png|jpg|jpeg|gif)["']?\)/g, function(match, file, type)
{
  var fileName = file + '.' + type;
  var size = fs.statSync(fileName).size;
  if (size > 4096) {
    console.log('Skipping ' + fileName + ' (' + (Math.round(size/1024*100)/100) + 'k)');
    return match;
  }
  else {
    var base64 = fs.readFileSync(fileName).toString('base64');
    if (typeof(files[fileName]) !== 'undefined') {
      console.log('Warning: ' + fileName + ' has already been base64 encoded in the css');
    }
    files[fileName] = true;
    return 'url("data:image/' + (type === 'jpg' ? 'jpeg' : type) + ';base64,' + base64 + '")';
  }
});

Edited to allow for single or double quotes.