so, you have an image that should change to another one from the net after it has been clicked?

make an array for the links:

PImage img;  // current image
String[] lnks;  // all links
int cimg = 0;  // pointer

void setup () {
  // ..
  lnks = new String[] {
"http://processing.org/img/processing_beta.gif"
// ...
};
  img = loadImage( lnks[cimg] );
 // ..
}

// ...

now once the image was clicked increment the pointer (make sure it is < lnks.length ) and load the next image ..

F