call_instagram.pde

import com.francisli.processing.http.*;

PFont InstagramFont;
PImage backgroundimg;
PImage userphoto;
PImage profilepicture;

String username;
String tag;
String[] tagStrings;

HttpClient client;

void setup() {
  size(600, 900);
  smooth();
  backgroundimg = loadImage("globalgram_background.jpg");


  InstagramFont = loadFont("Helvetica-Bold-36.vlw");

  client = new HttpClient(this, "api.instagram.com");
  client.useSSL = true;

  //// instantiate a new HashMap
  HashMap params = new HashMap();
  //// put key/value pairs that you want to send in the request
  params.put("access_token", "YOUR ACCESS TOKEN HERE");
  params.put("count", "1");
  client.GET("/v1/tags/nature/media/recent.json", params);
}

void responseReceived(HttpRequest request, HttpResponse response) {
  println(response.getContentAsString());

  //// we get the server response as a JSON object
  JSONObject content = response.getContentAsJSONObject();

  //// get the "data" value, which is an array
  JSONObject data = content.get("data");

  //// get the first element in the array
  JSONObject first = data.get(0);

  //// the "user" value is another dictionary, from which we can get the "full_name" string value
  println(first.get("user").get("full_name").stringValue());
  //// the "caption" value is another dictionary, from which we can get the "text" string value
  println(first.get("caption").get("text").stringValue());
  //// get profile picture
  println(first.get("user").get("profile_picture").stringValue());
  //// the "images" value is another dictionary, from which we can get different image URL data
  println(first.get("images").get("standard_resolution").get("url").stringValue());

  JSONObject tags = first.get("tags");
  tagStrings = new String[tags.size()];
  for (int i = 0; i < tags.size(); i++) {
    tagStrings[i] = tags.get(i).stringValue();
  }

  username = first.get("user").get("full_name").stringValue();

  String profilepicture_url = first.get("user").get("profile_picture").stringValue();
  profilepicture = loadImage(profilepicture_url, "png");

  String userphoto_url = first.get("images").get("standard_resolution").get("url").stringValue();
  userphoto = loadImage(userphoto_url, "png");

  //noLoop();
}


void draw() {
  background(255);
  imageMode(CENTER);
  image(backgroundimg, width/2, height/2);
  //rectMode(CENTER);
  //noStroke();
  //rect(70, 70, 90, 90);
  //fill(150);

  if (profilepicture != null) {
    image(profilepicture, 70, 70, 90, 90);
  }
  imageMode(CENTER);
  if (userphoto != null) {
    image(userphoto, width/2, height/2-40, 550, 550);
  }

  textFont(InstagramFont, 36);
  if (username != null) {
    text(username, 125, 90);
    fill(150);
  }

  textFont(InstagramFont, 14);
  if ((tagStrings != null) && (tagStrings.length > 0)) {
    String line = tagStrings[0];
    for (int i = 1; i < tagStrings.length; i++) {
      line += ", " + tagStrings[i];
    }
    text(line, 25, 690, 550, 50);
    fill(0);
  }
}

Paolo, this may help you :

http://instagram.com/developer/authentication/
Receiving an access_token

" In order to receive an access_token, you must do the following:

Direct the user to our authorization url.
If the user is not logged in, they will be asked to log in.
The user will be asked if they’d like to give your application access to his/her Instagram data.
The server will redirect the user in one of two ways that you choose:
Server-side flow (reccommended):Redirect the user to a URI of your choice. Take the provided code parameter and exchange it for an access_token by POSTing the code to our access_token url.
Implicit flow: Instead of handling a code, we include the access_token as a fragment (#) in the URL. This method allows applications without any server component to receive an access_token with ease."