data-url.css

gulpfile.js

var Data = require('datauri');
var path = require('path');

var inlineAssets = function(content) {
  var directory = path.dirname(content.source.input.file);

  content.replaceValues(/data-url\("(.*)"\)/, 'data-url(', function(value, match) {
    var image = new Data(path.join(directory, match));
    return 'url(' + image.content + ')';
  });
};

gulp.task('css', function() {
  return gulp.src('**/*.css')
    .pipe(postcss(autoprefixer(), inlineAssets]))
    .pipe(gulp.dest('dist/'));
});

sample.css

.foo {
  background: data-url("../images/bar.png") no-repeat;
}