public string PostUploadImage(string uploadUrl, string imgPath, string fileparameter = "file")
        {
            if (uploadUrl.StartsWith("https")) { System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls; }

            using (HttpClient httpClient = new HttpClient())
            {
                using (MultipartFormDataContent httpContent = new MultipartFormDataContent())
                {
                    using (var fileContent = new ByteArrayContent(System.IO.File.ReadAllBytes(imgPath))) 
                    {
                        int pos = imgPath.LastIndexOf("/");
                        string fileName = imgPath.Substring(pos + 1);
                        httpContent.Add(fileContent, fileparameter, fileName);
                        using (HttpResponseMessage response = httpClient.PostAsync(uploadUrl, httpContent).Result)
                        {

                            if (response.IsSuccessStatusCode)
                            {
                                string result = response.Content.ReadAsStringAsync().Result;
                                return result;
                            }
                            return null;
                        }
                    }
                        
                }

            }
        }

 调用：

PostUploadImage(URL 地址, 文件路径);