C# - WebMethod Caches Images Not Entering Handler?

I have a WebMethod that recives HTML as parameter somthing like this:

Public Function ConvertHtmlToPdfListAnswer(ByVal dokument As Dokument) As Byte()

In this HTML my path to for example user signature looks like this:

Handlers/SzablonyListImgHandler.ashx?usid=2006

My handler starts like this:

Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest

and it takes parameter from context, creates query for databese and returns stream output like this:

If Not String.IsNullOrEmpty(context.Request.QueryString("usid")) Then
...
...
streamOut.WriteTo(context.Response.OutputStream)

My problem is that it fires onle once...what I mean is that when I call the service again it doesn't enter the handler but it already shows me the image when i check the HTML in debugger view. So when The user changes his signature in database the service method still returns the previous one...