“从URL下载图像文件时，远程服务器返回错误（502）Bad Gateway”

I am trying to download image from website, but it gives me error i.e. (The remote server returned an error: (502) Bad Gateway). I search a lot on internet but didn't get exact solution. This code is working on some website but on few website it is showing above error. Please check my below code...I am trying to download image from website, but