Android: Which image format shall I use and why?

In android the best format of Image is PNG as it light compare to JPG,JPEG etc.So its easy to draw and take less time to perform the operation while using these images.

And For bitmap Go through this Bitmap Help

Edited Yes no need to use Bitmap ,if you have images in resource.If your images are in Drawable then use

ImageView img=new ImageView(this);
//You can use like that without using Bitmap
img.setImageResource(R.drawable.arrow_s);

Note that: "img.setImageResource(...)" still calls the bitmap routines, potentially on UI thread: http://developer.android.com/reference/android/widget/ImageView.html#setImageResource%28int%29

That may cause undesirable UI-lags, so you might want to deal with the BitmapFactory yourself, in a seperate thread, as discussed here: http://developer.android.com/training/displaying-bitmaps/index.html

As to the original question, PNG comes is many forms (see: 'PNG color options' and 'PNG color types' on the PNG Wiki page) -- Is there a favoured PNG format for Android?