Do somethings like this :
LayoutParams lp = new LayoutParams();
lp.gravity= Gravity.CENTER_HORIZONTAL;
myImg.setLayoutParams(lp);
UPDATE : Another way to do this :
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
params.weight = 1.0f;
params.gravity = Gravity.CENTER;
imageView.setLayoutParams(params);