Cannot Convert System.String to System.Uri

I am using the Web Client Class to download files from the internet (Flickr actually). This works fine as long as I use : WebClient().DownloadData(string) , however this locks up the UI as it is Not asynchronous.

However when I try WebClient().DownloadDatAsync(string), I get a compile error: "Unable to convert System.String to System.Uri".

The string MediumUrl returns "http://farm4.static.flickr.com/2232/2232/someimage.jpg"

So the question is how do I convert the string "http://farm4.static.flickr.com/2232/2232/someimage.jpg" to a Uri.

Things I have tried-

1. I have tried to cast it to Uri but that does not work either.

2. I have tried Uri myuri = new uri(string) - errors out as above.

   foreach (Photo photo in allphotos)  
   {  
       //Console.WriteLine(String.Format("photo title is :{0}", photo.Title));
       objimage = new MemoryStream(wc.DownloadData(photo.MediumUrl));
       images.Add(new Pictures(new Bitmap(objimage), photo.MediumUrl, photo.Title));  
   }