根据路径获得图片并压缩，返回bitmap用于显示

public static Bitmap getSmallBitmap(String filePath) {
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;// 设置为ture,只读取图片的大小，不把它加载到内存中去
BitmapFactory.decodeFile(filePath, options);

// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, 480, 800);// 此处，选取了480x800分辨率的照片

// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;// 处理完后，同时需要记得设置为false

return BitmapFactory.decodeFile(filePath, options);
}