PImage[] myImages;  // the array
int currentImage = 0;   // the index, pointing to the first image at 0

void setup () {
   // ...
   // initialize array
   myImages = new PImage[/* amount of images goes here */];
   // load images
   myImages[0] = loadImage( "image1.jpg" );
   myImages[1] = loadImage( "image2.jpg" );
   myImages[2] = loadImage( "image2.jpg" );
   // ...
}

void draw () {
    image( myImages[currentImage], 0, 0 );
    // ...
    if ( /* sonia code to check level == 100 */ )
    {
	  currentImage++; // increment index
	  currentImage %= myImages.length;  // make sure its >= 0 and < array.length
	  // read up on modulo "%"
	  // http://processing.org/reference/modulo.html
    }
}