Look here ( http://www.lagers.org.uk/g4ptool/index.html ) at the GUI Builder. Check out the video guides.

Look under tools in Processing. The videos show how to make a button from an image, etc. It is part of Processing. What the videos.

Yes, that is what it does. It is for building the GUI. You will need to add your own code for everything else in the Processing IDE.

Using a library is a solution, but for the expressed needs, a simple image() call, with detection of click within the rectangle of the image can be enough. There are examples shipped with Processing showing such button (without the image, but it is simple to adapt).

Add a PImage field to this class, and use loadImage() in the constructor to initialize it.
Then in the display() method (or whatever it is called), replace a probable rect() call by an image() one.
Supposing the image is of the same size than the button, of course.

Remarks:
- Don't use special variables to define width and height of sketch! Just define them in the size() call, and use the predefined variables width and height.
- Don't use Boolean, use boolean (lower case).
- Use boolean as a real type: instead of

1.    if(mouseX > xPos && mouseX < xPos+bWidth && mouseY > yPos && mouseY < yPos+bHeight){    
2.     isMouseOver = true;
3.   } else {
4.     isMouseOver = false;
5.   }

write:

1.    isMouseOver = mouseX > xPos && mouseX < xPos+bWidth && mouseY > yPos && mouseY < yPos+bHeight;

- Avoid using == with strings, use equals() instead (see the Reference page on String), it will avoid subtle errors later.
- I don't see my advice followed in your code...

1. class IButton {
2.  // Declarations there
3.   // New field
4.   PImage buttonImage;
5.   // Constructor
6.   IButton(String pID, int pX, int pY){
7.     // Other inits here
8.     buttonImage = loadImage("button.png");
   
9.   }
   

What do you not understand in my last sample? It just shows where to place the field and how to load the image (change its name if needed, of course). And add an image(buttonImage, ...) call in the update method (instead of these fill() and rect() calls).

" it came up as an error."
We cannot guess what is this error... Show the current state of your code.