429 too many requests error even though I paid

[mykorean...@gmail.com]

I have got subscribed that costs 25 $ per an month.

But I still don't have permission to upload photos more than 20 photos

I just want to upload photos from url without any limit.

I'm a c# developer and want to know how to get the key for uploading and how to upload photos from url

The source code is below
using (WebClient client = new WebClient())
{
client.Headers.Add("Authorization", "Client-ID " + key);
client.Headers.Add("X-Mashape-Key", mashape_key);
string response = client.DownloadString("http://imgur.com/upload?url=" + old_img_url);

It works for few uploadings but it returns 429 too many requests error later

Thank you