You can also try this, suppose if you want to make a back image button and you have "500x500 png" and want it to fit in small button size.
add this line of code to your Imageview.
android:scaleType="fitXY"
EXAMPLE:
<ImageView
android:layout_width="50dp"
android:layout_height="50dp"
android:id="@+id/imageView2"
android:src="@drawable/Backicon"
android:scaleType="fitXY"
/>