So you want to split the screenImage? 

yes - i tried it with get(x,y,width, height) method - the problem is, that it doesn't work with the continously numbers saving.
i have no idea, why it doesn't work with the numbers. the way it is programmed below it overwrites the previous picture.
any idea why?

1. PImage partialSave = get(0,0,width/2,height);
   partialSave.save("partialSave-###.png");
   

The ### is replaced with the number of the current frame. If you do two saveFrame() in the same frame, you will get the same number, indeed.
If you need to manage your own numbering, you can do it with:
partialSave.save("partialSave-" + nf(counter++, 4) + ".png");
where counter is a global variable you created.

thanks for your help!
i tried it as you said - that only works, when i keep the screen open. what i need is, that i can do a new «run» because i need to load a new picture from the camera for each saved picture.

the problem with the new «run» is, that it overwrites always number picture0000.png - which means, that i have to press 5 times the s-key to get picture number 5 - but with the 5 times pushing the s-key, i overwrite  0 to 4.
what can i do, that it saves one higher than the last one? counting somehow backwards?

Then you at each run, you have to check, what files are already there. like this:

Ah, in this case, you need to save the number of the last saved image in a file when closing, and load it back when running again. Thus, you can resume the numbering where you left.
Alternatively, you can list the files, extract the number from the file name, and find out the maximum value. Then you can resume at this value + 1.

ramin - i tried your script - it does only save the wohle picture and still ignores the counting for only one part. it seems as it would ignore the counting as soon as a variable is before the command save...

and philho - sounds good - but - how ???

How, what? I proposed two alternatives.
For the first one, look at saveStrings() and loadStrings().

after a long time trying - it finally works with the help of ramin's script - thank's a lot!
here is the working script

1.    import java.io.File; 
   
      int i=0;
      boolean exists=true;
    
      void setup(){
      size(400,200);
    
    
    }
      void draw(){
                 
         if(mousePressed){
         //right side 
         stroke(255,0,0);
         strokeWeight(1);
         //left side    
         line(mouseX, mouseY, pmouseX, pmouseY);
         stroke(0);
         line(mouseX-width/2, mouseY, pmouseX-width/2, pmouseY);
         }
    
      }
    
     void keyPressed(){
         PImage a = get(0,0,width/2,height);   
           if (key == 's') {
             while (exists) {
               exists = new File(sketchPath+"/a"+i+".png").exists();
               i++;
             }     
           i--;
           a.save("partialSave"+i+".png");
           save("a"+i+".png");
           println(i);
    
         }
     }