Ok, to be honest, this looks like a little bit of copy&paste.
"folder.list(jpgFilter)" returns an array of filenames. The length of this array is the number of jpg-images in your folder.
No need for converting this array to an int-array or something.

And you should maybe just keep that filename-array for later use when loading the images, so you don't have to name your files "1.jpg", "2.jpg", etc.

Something like that:

1. String[] fileNames;
   
   void setup() {
     // have a look in the data folder
     java.io.File folder = new java.io.File(dataPath(""));
   
     // this is the filter (returns true if file's extension is .jpg)
     java.io.FilenameFilter jpgFilter = new java.io.FilenameFilter() {
       public boolean accept(File dir, String name) {
         return name.toLowerCase().endsWith(".jpg");
       }
     };
   
     // get a list of jpg-files in the data folder
     fileNames = folder.list(jpgFilter);
     println(fileNames.length + " jpg files in specified directory");
   
     // load random jpg-image, if there are any
     if (fileNames.length > 0) {
       int picNumber = ceil(random(0, fileNames.length));
       PImage img = loadImage(fileNames[picNumber]);
       image(img, 0, 0);
     }
   }
   
   

Benja!

It will be easier with the code :)

Arrays start with index 0, so the last index of an array with 1223 items is 1222.
it was my fault, you shouldn't use ceil(), try floor() instead.

1.         giftoDisplay = floor(random(0, numberofGif.length));

Perfect!