I use the following in my
Silverlight code
1 string host = App.Current.Host.Source.ToString();2 host = host.Substring(0, host.IndexOf("/ClientBin"));3 WebClient localwebclient = new WebClient();4 localwebclient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(localwebclient_DownloadStringCompleted);5 localwebclient.DownloadStringAsync(new Uri(String.Format("{0}/{1}", host, DATAFILE)));
Obviously this is for data file downloads but the DownloadStringAsync takes a URI which is the bit you are looking for, last line.
Just change the String.Format to
String.Format("{0}/{1}/{2}",host, DATAFOLDER, IMAGEFILE)
setting DATAFOLDER to your data folder and IMAGEFILE to your image.