ImageRequest的用法：

1.發出http請求，需要傳入六個參數(url,success response,width,height,顏色屬性,error response)。

ImageRequest imageRequest = new ImageRequest(  
        url,  
        new Response.Listener<Bitmap>() {  
            @Override  
            public void onResponse(Bitmap response) {  
                imageView.setImageBitmap(response);  
            }  
        }, 0, 0, Config.RGB_565, new Response.ErrorListener() {  
            @Override  
            public void onErrorResponse(VolleyError error) {  
                imageView.setImageResource(R.drawable.default_image);  
            }  
        });

2.加入RequestQueue

mQueue.add(stringRequest);