Try with the following code to get all images from SlideShow folder and its sub folder
public static ArrayList<String> getPathOfAllImages(Activity activity) {
        trimCache();
        String absolutePathOfImage = null;
        String nameOfFile = null;
        String absolutePathOfFileWithoutFileName = null;
        Uri uri;
        Cursor cursor;
        int column_index;
        int column_displayname;
        int lastIndex;
        // absolutePathOfImages.clear();
        ArrayList<String> absolutePathOfImageList = new ArrayList<String>();
        if (absolutePathOfImageList.isEmpty()) {
            uri = android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
            String[] projection = { MediaColumns.DATA,
                    MediaColumns.DISPLAY_NAME };
            cursor = activity.managedQuery(uri, projection, null, null, null);
            column_index = cursor.getColumnIndexOrThrow(MediaColumns.DATA);
            column_displayname = cursor
                    .getColumnIndexOrThrow(MediaColumns.DISPLAY_NAME);
            // cursor.moveToFirst();
            while (cursor.moveToNext()) {
                // for(int i=0; i<cursor.getColumnCount();i++){
                // Log.i(TAG,cursor.getColumnName(i)+".....Data Present ...."+cursor.getString(i));
                // }
                // Log.i(TAG,"=====================================");
                absolutePathOfImage = cursor.getString(column_index);
                nameOfFile = cursor.getString(column_displayname);
                lastIndex = absolutePathOfImage.lastIndexOf(nameOfFile);
                lastIndex = lastIndex >= 0 ? lastIndex
                        : nameOfFile.length() - 1;
                absolutePathOfFileWithoutFileName = absolutePathOfImage
                        .substring(0, lastIndex);
                if (!((absolutePathOfFileWithoutFileName.equals(Environment
                        .getExternalStorageDirectory() + "/SlideShow/")))) {
                    if (absolutePathOfImage != null) {
                        absolutePathOfImageList.add(absolutePathOfImage);
                    }
                }
            }
        }
        // Log.i(TAG,"........Detected images for Grid....."
        // + absolutePathOfImageList);
        return absolutePathOfImageList;
    }