Updated code. I got an email earlier with a response I didn't quite understand. I think the user name was Phi.lo? But for some reason now I can't find it.

Here is what my code looks like now:
// The base URL for Twitter's Search API
PImage img;
String api  = "http://api.flickr.com/services/rest/?method=flickr.tags.getClusterPhotos&";


void setup() {
  size(800, 800);
 
  //text - Free text search. Searches the title, description or tags that contain this text.
  //page - Which page of results you want returned
  //per_page - Number of photos to return per page
  //api_key - Required to access API
 
  String query = api + "tag=cat&cluster_id=1&format=rest&extras=url_o&api_key=bd7435660785b923332b224789d33c93";
  println("Our final query URL:" + query);
  println("**********************");
 
  XMLElement xml = new XMLElement(this, query);
  XMLElement displayPhoto = xml.getChild("photos/photo/url_o=");
 
  PImage photo = loadImage( displayPhoto.getContent() );
   image(photo, 0, 0, width, height);
  println(xml);
 
}

I get a NullPointerException error when it's run though. I'm not quite sure how to access the information within a child in the XML document. Still trying to figure that one out.. I also need to have one photo show at a time and then switch to the next one

I've been meaning to work on a simple way to return flickr images to use with various sketches I am working on so I wrote the following which you could easily adapt. It returns a string which you can simply use in an image method:

I had a problem a long time ago where processing wasn't able to connect to the web because the network I was on required a web proxy server to access the web, and it was screwing up the flickr lookups. Maybe that's the problem. 

I've made some progress over the last few days and I have a code that will display the last image in an array of images. See below:
           

1. String api  = "http://api.flickr.com/services/rest/?method=flickr.tags.getClusterPhotos&";
   
   
   void setup() {
     size(800, 800);
   
     //text - Free text search. Searches the title, description or tags that contain this text.
     //page - Which page of results you want returned
     //per_page - Number of photos to return per page
     //api_key - Required to access API
   
     String query = api + "tag=cat&cluster_id=1&format=rest&extras=url_o&api_key=YOURAPIKEY";
     println("Our final query URL:" + query);
     println("**********************");
   
     XMLElement xml = new XMLElement(this, query);
     println(xml);
   
     XMLElement[] photoTag = xml.getChildren("photos/photo");
     for (int i = 0; i < photoTag.length; i++) {
   //println(displayPhoto);
   
     String farm = photoTag[i].getString("farm");
     String server = photoTag[i].getString("server");
     String id = photoTag[i].getString("id");
     String secret = photoTag[i].getString("secret"); 
     String img = "http://farm"+farm+".static.flickr.com/"+server+"/"+id+"_"+secret+".jpg";
     println(img);
     PImage photo = loadImage( img );
     image(photo, 0, 0, width, height);
     }
   }

But I need something that displays each image in the array one by one, this code counts through the 24 images in the array but then displays them all at once on top of one another... I know it's probably a stupid easy fix that I'm just not seeing after staring at it for hours. Any suggestions? You guys are awesome so far and super helpful!

1. int maxImages = 24;
   int imageIndex = 0;
   PImage[] photo = new PImage[maxImages];
   
   String api  = "http://api.flickr.com/services/rest/?method=flickr.tags.getClusterPhotos&";
   
   
   void setup() {
     size(500, 500);
   
     //text - Free text search. Searches the title, description or tags that contain this text.
     //page - Which page of results you want returned
     //per_page - Number of photos to return per page
     //api_key - Required to access API
   
     String query = api + "tag=cat&cluster_id=1&format=rest&extras=url_o&api_key=YOURAPIKEY";
     println("Our final query URL:" + query);
     println("**********************");
   
     XMLElement xml = new XMLElement(this, query);
     println(xml);
   
     XMLElement[] photoTag = xml.getChildren("photos/photo");
     for (int i = 0; i < photoTag.length; i++) {
   //println(displayPhoto);
   
     String farm = photoTag[i].getString("farm");
     String server = photoTag[i].getString("server");
     String id = photoTag[i].getString("id");
     String secret = photoTag[i].getString("secret"); 
     String img = "http://farm"+farm+".static.flickr.com/"+server+"/"+id+"_"+secret+".jpg";
     println(photo);
     photo[i] = loadImage( img );
     image(photo[imageIndex], 0, 0);
     imageIndex = (imageIndex + 1) % photo.length;
     }
     frameRate(5);
   }