Which libraries are you using?
Can you post a simple piece of code demonstrating the problem?

Change the main tab to this

1. import g4p_controls.*;
2. 
   
3. void setup(){
4.   size(1000,600);
5.   createGUI();
6. }
7. 
   
8. void draw(){
9.   background(255);
10. }

I have just tried creating an image button and it worked fine - the jpg images showed up in the GUI Builder and in the final sketch.

Was there an error message in the black box under you code in the Processing editor - if so copy and paste it into this forum.

The problem is that Processing (or more accurate Java) does not recognize the files as jpg image files.

When you saved them as jpgs in Photoshop I suspect you simply typed in the name "button1.jpg" but this would not save the images as jpg.

Not familiar with Photoshop but you either have to export the image as jpg or in the save dialog box there maybe a dropdown list of file types and you choose jpg.

Use a boolean variable to determine which of those 2 images to display.
Another option is to have another PImage variable to refer to either start or stop PImages.
Toggle them when a click is detected inside its area.

Look into using the GToggleImageButton