Validating existence of image URL in report

I'm trying to validate the existence of an image URL for a report page in SQL Server Reporting Services.  The image URL is dynamically created based on an item number.  I either display the found image at the URL or a special image that indicates there are is no image for the item.  All of the images are on a virtual directory on the same server as the report server.

I've tried specifying the folder path \\servername\... and that does not work (as I would expect).  I also tried specifying custom code in the report using a programmatic web request (HttpWebRequest).  I expected that, but when I use it I get the following exception:

System.Security.SecurityException: Request for the permission of type 'System.Net.WebPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.
at System.Security.CodeAccessSecurityEngine.Check(Object demand, StackCrawlMark& stackMark, Boolean isPermSet)
at System.Security.CodeAccessPermission.Demand()
at System.Net.HttpWebRequest..ctor(Uri uri, ServicePoint servicePoint)
at System.Net.HttpRequestCreator.Create(Uri Uri)
at System.Net.WebRequest.Create(Uri requestUri, Boolean useUriBase)
at System.Net.WebRequest.Create(String requestUriString)
at ReportExprHostImpl.CustomCodeProxy.UrlExists(String u)
The action that failed was:
Demand
The type of the first permission that failed was:
System.Net.WebPermission
The Zone of the assembly that failed was:
MyComputer

This works in Visual Studio but does not work on the report server or another server on the network.  I need a method that works to perform this validation or work around it so that the special image is shown if the other image is missing.