Patterns are NOT PImage, so you can't cast one into the other, whatever you try.
Why did you abandon to put the PImages in the Patterns class? It was a good idea, I just advised to remove the "class PImage" declaration inside as it was serving no purpose, rather hiding the real PImage declaration.

The problem I am having is that I just don't know how to do it intelligently. I need my PImages to belong to a class called Patterns. Given that these patterns are clip art, the only way I can load them is by using PImages, but after that ...I don't know how to efficiently deal with the fact that I want to #1 represent them as thumbnails on the left of my screen; #2 be able to rotate them 90 degrees by clicking on them and pressing the R key; #3 be able to possible give them a random color by clicking on them (as I have been able to do with my color box and my stitches box). #4 copy them into the quilt grid to make a quilt....and possibly other functions. Here is what I have right now (this half my program, I'll send the other half which runs fine in reply to your other message).


PFont fontInstructions; // create a variable to load the font

PImage [] quiltPatterns = new PImage [4]; // declare a variable that stores an array of quilting patterns images from clip art


EmptyGrid myEmptyGrid;
 

void setup() {  // call the setup function to define initial enviroment properties

  size(900, 700);  // set the canvas size at 800 pixels by 800 pixels
  background(230); // set background color to light gray
 
  fontInstructions=loadFont("CurlzMT-48.vlw");  // load the font used for the text in a data file attached to this file
  textFont (fontInstructions);  // set the loaded font as the current one to use
 
 
 

  myEmptyGrid= new EmptyGrid (color(227, 208, 140), 100);

  for (int q = 0; q < quiltPatterns.length; q++) {  // create an array that loads the array of quilt patterns as numbered files
    quiltPatterns[q] = loadImage("pattern" + (q+1) + ".gif");  // load each quilt pattern individually based on its numbered file
  }  // end the for loop for loading the array of quilt patterns as numbered files
}  // end the setup function



void draw () {  // call the draw function

  background(230); // reset background color to light gray everytime the draw function is recalled

  image (quiltPatterns[0], 0, height-600, 100, 100);
  image (quiltPatterns[1], 0, height-450, 100, 100);
  image (quiltPatterns[2], 0, height-300, 100, 100);
  image (quiltPatterns[3], 0, height-150, 100, 100);

  myEmptyGrid.display();

  }  // end the draw function
 
 
  abstract class Grid {

  float sizeOfSquares;
  color gridClr;
 

  Grid(color tempgridClr, float tempsizeOfSquares) {

    gridClr=tempgridClr;
    sizeOfSquares=tempsizeOfSquares;
   
   
  }  // end of the constructor


  void display () {

    // initialization of quilt patterns (from Shiffman page 261)

    for (float xPosSquare=0;xPosSquare<width;xPosSquare+=sizeOfSquares) {
      for (float yPosSquare=0;yPosSquare<height;yPosSquare+=sizeOfSquares) {
        fill(gridClr);
        rect(xPosSquare, yPosSquare, sizeOfSquares, sizeOfSquares);
      }
    }
  }  // end of the display function
}


class EmptyGrid extends Grid {


  EmptyGrid(color tempgridClr, float tempsizeOfSquares) {
   
super (tempgridClr, tempsizeOfSquares);
       
    gridClr=tempgridClr;
    sizeOfSquares=tempsizeOfSquares;
   
  }  // end of the constructor

 
   void display () {

    // initialization of quilt patterns (from Shiffman page 261)

    for (int i=200;i<width;i+=sizeOfSquares) {
      for (int j=100;j<height;j+=sizeOfSquares) {
        fill(gridClr);
        rect(i, j, sizeOfSquares, sizeOfSquares);
      }
    }
  }  // end of the display function
   
   
}  // end of the Grid class


class Patterns extends Grid {

 
Patterns(color tempgridClr, float tempsizeOfSquares) {
   
super (tempgridClr, tempsizeOfSquares);
       
    gridClr=tempgridClr;
    sizeOfSquares=tempsizeOfSquares;
   
  }  // end of the constructor
 
}