You can do it with scale. If you remember to also translate and scale back before doing other stuff.

Try something likes this:

1. PImage img = loadImage("image.png");
2. // Flip about x-axis
   
3. pushMatrix();
   
4. translate(0, img.height);
   
5. scale(1, -1);
   
6. image(img, 0, 0);
   
7. popMatrix();
8. //Flip about y-axis
9. pushMatrix();
   
10. translate(0, img.height);
    
11. scale(1, -1);
    
12. image(img, 0, 0);
    
13. popMatrix();
14. // Flip around both
15. pushMatrix();
    
16. translate(img.width, img.height);
    
17. scale(-1, -1);
    
18. image(img, 0, 0);
    
19. popMatrix();