Can you post your code?  I've seen this with various sketches, and sometimes it depends on the browser - it will work in Chrome or Safari but not Firefox (or vice versa).

Currently, only Firefox and other Mozilla-based browsers 

2. PImage one;
3. PImage two;
4. PImage three;
5. PImage four;
6. PImage five;
7. PImage six;
8. PImage seven;
9. PImage eight;
10. PImage nine;
11. PImage ten;
12. PImage eleven;
13. PImage twelve;
14. PImage thirteen;
15. PImage fourteen;
16. PImage fifteen;
17. PImage sixteen;
18. PImage seventeen;
19. 
    
20. int counter=1;
21. 
    
22. void setup(){
23.   one=loadImage("1.jpg");
24.   two=loadImage("2.jpg");
25.   three=loadImage("3.jpg");
26.   four =loadImage("4.jpg");
27.   five=loadImage("5.jpg");
28.   six =loadImage("6.jpg");
29.   seven=loadImage("7.jpg");
30.   eight=loadImage("8.jpg");
31.   nine=loadImage("9.jpg");
32.   ten= loadImage("10.jpg");
33.   eleven=loadImage("11.jpg");
34.   twelve=loadImage("12.jpg");
35.   thirteen=loadImage("13.jpg");
36.   fourteen=loadImage("14.jpg");
37.   fifteen=loadImage("15.jpg");
38.   sixteen=loadImage("16.jpg");
39.   seventeen=loadImage("17.jpg");
40.   
41.  size(1200,800); 
42. }
43. 
    
44. void draw(){
45.   if(counter==1){
46.     fill(0);
47.     rect(0,0,1200,800);
48.   image(one,width/3,height/3);
49.   }
50. 
    
51. if(counter==2){
52.  image(two,0,0); 
53.   
54. }
55. if(counter==3){
56.   fill(0);
57.   rect(0,0,1200,800);
58.  image(three,width/4,height/4); 
59.   
60. }
61. if(counter==4){
62.     fill(0);
63.   rect(0,0,1200,800);
64.  image(four,width/3,height/3); 
65.   
66. }
67. if(counter==5){
68.     fill(0);
69.   rect(0,0,1200,800);
70.  image(five,width/3,height/3); 
71.   
72. }
73. if(counter==6){
74.     fill(0);
75.   rect(0,0,1200,800);
76.  image(six,0,0); 
77.   
78. }
79. if(counter==7){
80.     fill(0);
81.   rect(0,0,1200,800);
82.  image(seven,width/3,height/3); 
83.   
84. }
85. if(counter==8){
86.     fill(0);
87.   rect(0,0,1200,800);
88.  image(eight,0,0); 
89.   
90. }
91. if(counter==9){
92.     fill(0);
93.   rect(0,0,1200,800);
94.  image(nine,width/3,height/3); 
95.   
96. }
97. if(counter==10){
98.     fill(0);
99.   rect(0,0,1200,800);
100.  image(ten,width/3,height/3); 
101.   
102. }
103. 
     
104. if(counter==11){
105.     fill(0);
106.   rect(0,0,1200,800);
107.  image(eleven,0,0); 
108.   
109. }
110. 
     
111. if(counter==12){
112.     fill(0);
113.   rect(0,0,1200,800);
114.  image(twelve,0,0);
115.   
116. }
117. 
     
118. if(counter==13){
119.     fill(0);
120.   rect(0,0,1200,800);
121.  image(thirteen,width/3,height/3);
122.   
123. }
124. 
     
125. if(counter==14){
126.     fill(0);
127.   rect(0,0,1200,800);
128.  image(fourteen,width/3,height/3);
129.   
130. }
131. 
     
132. if(counter==15){
133.     fill(0);
134.   rect(0,0,1200,800);
135.  image(fifteen,width/3,height/3);
136.   
137. }
138. 
     
139. if(counter==16){
140.     fill(0);
141.   rect(0,0,1200,800);
142.  image(sixteen,0,0);
143.   
144. }
145. 
     
146. if(counter==17){
147.     fill(0);
148.   rect(0,0,1200,800);
149.  image(seventeen,width/3,height/3);
150.   
151. }
152. 
     
153. if(counter==18){
154.   fill(0);
155.   rect(0,0,1200,800);
156.   counter=1;
157.   
158. }
159. }
160.  void mouseReleased(){
161.     
162.     counter=counter +1;

  }

Repeating: Most browsers decided to suppress code running from local files for some time ago!

Third topic you created around the same problem...
This one belongs here (moved), not to Programming Questions, since it is specific to JS.

About your program, I will repeat an advice I give at least once a day... Put the size() call at the top of setup().
Have you read about PJS? All images must be pre-loaded for the sketch to load them. There is a special instruction for that.

That's odd that it works in Firefox given that as PhiLho pointed out, you do not have what is known in PJS as a "directive" to load the images:

Perhaps, the @pjs preload directive only helps to have an image ready to load.

good point.  right.  maybe it doesn't break the sketch all together.