<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    android:orientation="vertical">

    <ImageView
        android:id="@+id/image"
        android:layout_width="wrap_content"
        android:layout_height="240sp"
        android:src="@drawable/lijiang" />
    
    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="57dp" 
        android:max="255"
        android:progress="255"
        android:thumb="@drawable/ic_launcher"/>

</LinearLayout>

package com.example.seekbar;

import android.app.Activity;
import android.os.Bundle;
import android.widget.ImageView;
import android.widget.SeekBar;
import android.widget.SeekBar.OnSeekBarChangeListener;

public class MainActivity extends Activity {

    ImageView image;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        image=(ImageView) findViewById(R.id.image);
        SeekBar seekBar=(SeekBar) findViewById(R.id.seekBar);
        seekBar.setOnSeekBarChangeListener(new OnSeekBarChangeListener(){

            @Override
            public void onProgressChanged(SeekBar seekBar, int progress,
                    boolean fromUser) {
                image.setImageAlpha(progress);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {
            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {
            }
            
        });
    }
}