UploadWithMedia.gs

function oAuthConfig() {
  var oAuthConfig = UrlFetchApp.addOAuthService("twitter");
  oAuthConfig.setAccessTokenUrl("http://api.twitter.com/oauth/access_token");
  oAuthConfig.setRequestTokenUrl("http://api.twitter.com/oauth/request_token");
  oAuthConfig.setAuthorizationUrl("http://api.twitter.com/oauth/authorize");
 
  // Register an app at https://dev.twitter.com/apps/new to get the following key and secret
  oAuthConfig.setConsumerKey("key");
  oAuthConfig.setConsumerSecret("secret");
}
 
function postImage() {
 
  oAuthConfig();
 
  var boundary = "cuthere";
  var picture = UrlFetchApp.fetch("https://twitter.com/images/resources/twitter-bird-white-on-blue.png").getBlob().setContentTypeFromExtension();
  var status = "Test upload with media";

  var requestBody = Utilities.newBlob("--"+boundary+"\r\n"+
  "Content-Disposition: form-data; name=\"status\"\r\n\r\n"+status+"\r\n"+
  "--"+boundary+"\r\n"+
  "Content-Disposition: form-data; name=\"media[]\"; filename=\""+picture.getName()+"\"\r\n"+
  "Content-Type: "+picture.getContentType()+"\r\n\r\n").getBytes();
  requestBody = requestBody.concat(picture.getBytes());
  requestBody = requestBody.concat(Utilities.newBlob("\r\n--"+boundary+"--\r\n").getBytes());
  
  var options =
  {
    method: "post",
    contentType: "multipart/form-data; boundary="+boundary,
    oAuthServiceName: "twitter",
    oAuthUseToken: "always",
    payload: requestBody
  };
  var request = UrlFetchApp.fetch("https://api.twitter.com/1.1/statuses/update_with_media.json", options);
}