options.inSampleSize = scale;

/**
 * 压缩图片
 * @author chen.lin
 *
 */
public class LoadImageActivity extends Activity implements OnClickListener {
  private Button mBtnLoad;
  private ImageView mImageView;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_image_load);
    initViews();
  }
  private void initViews() {
    mBtnLoad = (Button) findViewById(R.id.btnLoadImage);
    mImageView = (ImageView) findViewById(R.id.imageView);
    mBtnLoad.setOnClickListener(this);
  }
  @Override
  public void onClick(View v) {
    if (v == mBtnLoad) {
      Options options = new Options();
      BitmapFactory.decodeFile("/sdcard/images/1.jpg", options);
      //不去真的解析图片，只是获取图片的头部信息，宽高
      options.inJustDecodeBounds = true;
      //得到图片的真实宽高
      int imageHeight = options.outHeight;
      int imageWidth = options.outWidth;
      //得到屏幕的宽高
      WindowManager wm = (WindowManager) getSystemService(WINDOW_SERVICE);
      int screenHeight = wm.getDefaultDisplay().getHeight();
      int screenWidth = wm.getDefaultDisplay().getWidth();
      //得到缩放比例
      int scale = 1;
      int scaleX = imageWidth / screenWidth;
      int scaleY = imageHeight / screenHeight;
      if (scaleX > scaleY & scaleX >=1) {//表示如果宽的缩放比例大于高的，并且scaleX>=1都为true
        scale = scaleX;
      }
      if (scaleY > scaleX & scaleY >=1) {//表示如果高的缩放比例大于宽的，并且scaleY>=1都为true
        scale = scaleY;
      }
      //解析图片
      options.inJustDecodeBounds = false;
      //修改图片的缩放比例，如果scale=4说明图片缩小4倍，像数=1/16
      options.inSampleSize = scale;
      Bitmap bm = BitmapFactory.decodeFile("/sdcard/images/1.jpg", options);
      mImageView.setImageBitmap(bm);
    }
  }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical" >
  <ImageView
    android:id="@+id/imageView"
    android:layout_width="fill_parent"
    android:layout_height="0dp"
    android:layout_weight="1" />
  <EditText
    android:id="@+id/editText1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:ems="10" >
    <requestFocus />
  </EditText>
  <Button
    android:id="@+id/btnLoadImage"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:onClick="loadImage"
    android:text="加载图片" />
</LinearLayout>