// i'll do this for one image, so you see the concept
// hope it works.. just off the top of my head
//
// first, you need a stored imageposition
int imgX, imgY;
// and an image
PImage img;

// mouseposition offset to image
int mX, mY;

// regular old setup
void setup()
{
 size(800,600);
 img = loadImage("img.jpg");
 centerImage();
}

// draw method
void draw()
{
 background(0);
 if(mousePressed){ // is the mousebutton being held?
   imgX = mouseX-mX;
   imgY = mouseY-mY;
 }
 image(img,imgX,imgY);
}

// mousepressed
void mousePressed()
{
 // set variables for holding mouseposition offset
 // to the image
 mX = mouseX-imgX;
 mY = mouseY-imgY;
}

// any key pressed, will re-center the image
void keyPressed()
{
centerImage();
}

// center image function
void centerImage()
{
 imgX = (width-img.width)/2;
 imgY = (height-img.height)/2;
}