Yes we can! Make a method of the same name as your button. That code will be executed when the button is pressed. You can put the screenshot code in there.

Sidenote: depending on your exact code, this will create a screenshot WITH the controlP5 controllers (like the example below). Sometimes this is exactly what you want. Sometimes you want a screenshot WITHOUT the controlP5 controllers in it. Then you have to set controlP5 autoDraw to false, toggle a boolean with the button/toggle and put your screenshot code in the if (screenShotBoolean==true) check at the end of draw(). You can put the manual controlP5 draw() after that.

Code Example

1. import controlP5.*;
2. ControlP5 cp5;
3.  
4. void setup() {
5.   size(400, 600);
6.   background(0);
7.   smooth();
8.   cp5 = new ControlP5(this);
9.   cp5.addButton("saveScreen").setPosition(100, 100).setSize(200, 100);
10. }
11.  
12. void draw() {
13.   fill(random(255), random(255), random(255));
14.   ellipse(random(width), random(height), 100, 100);
15. }
16.  
17. public void saveScreen() {
18.   saveFrame();
19. }