You have already an array of image names. Declare an array of images, and load them in setup(), to avoid loading them over and over.
Next, you can probably compute the positions, to avoid copy & paste: let the computer to work, not you! I see mouseX is tested by increments of 100 pixels, and mouseY against three values separated by 200 pixels. Also factor out common code: code that is in the three ifs can be put outside (before it). Code common to the last two ifs (on each case) can be done before testing the second case. Etc.

Mmm, to be more concrete, here is a way (untested) to do what I say:

1. // Somewhere before setup()
2. int PERSO_NB = 50;
3. int INTERVAL_X = 100;
4. int INTERVAL_Y = 200;
5. PImage[] persoImages = new PImage[PERSO_NB];
6. PImage[] persoTitleImages = new PImage[PERSO_NB];
7. PImage[] persoFunctionImages = new PImage[PERSO_NB];
8.  
9. for (int i = 0; i < PERSO_NB; i++) {
10.   if (mouseX <= (i + 1) * INTERVAL_X) {
11.     image(getPersoImage(i), 0, 0);
12.     if (mouseY <= 2 * INTERVAL_Y) {
13.       image(getPersoTitleImage(i), 0, 0);
14.       if (mouseY <= 3 * INTERVAL_Y) {
15.         image(getPersoFonctionImage(i), 0, 300);
16.       }
17.     }
18.     break;
19.   }
20. }
21.  
22. PImage getPersoImage(int n) {
23.   if (persoImages[n] == null) { // Not loaded yet
24.     persoImages[n] = loadImage(perso[n]);
25.   }
26.   return persoImages[n];
27. }
28. // Idem for getPersoTitleImage and getPersoFunctionImage

I added a function (you must have three of them) to load lazily, with a cache, the images: they are loaded only when needed, but once loaded, you won't access disk again.