I have changed my code like below, than i have downloaded the on demand confluence page content images to my local machine
image=https://{companyName}.atlassian.net/wiki/download/attachments/524312/worddave1f7873c424d7824e580764369e7ee68.png;
imageLocation =@"C:\Images";
string[] FileName = image.Split(new char[] { '/' }, StringSplitOptions.RemoveEmptyEntries);
WebClient client = new WebClient();
client.DownloadFile(new Uri(image+"?&os_username=xxx&os_password=yyy"), imageLocation + "\\" + FileName[FileName.Count() - 1]);