Question on the slideshow extender

I need to set up the slide show based on a querystring parameter.  I thought I had this figured out, but I clearly don't understand something about how the webservice works. 

I have this in the .aspx page:

<asp:Button ID="btnPrev" runat="server" Text="Prev" />
    <asp:Button ID="btnPlay" runat="server" Text="Play" />
    <asp:Button ID="btnNext" runat="server" Text="Next" />
    <asp:Label ID="lblDesc" runat="server" />
    <cc1:SlideShowExtender ID="SlideShowExtender1" runat="server" BehaviorID="SSBehaviorID"
            TargetControlID="tstSlides"
            AutoPlay="true"
            ImageDescriptionLabelID="lblDesc"
            NextButtonID="btnNext"
            PreviousButtonID="btnPrev"
            PlayButtonID="btnPlay"
            PlayButtonText="Play"
            StopButtonText="Stop"
            Loop="true" >
        </cc1:SlideShowExtender>
    <asp:Image ID="tstSlides" runat="server" height="300px" Width="300px" />

And this in the code behind:

        Dim smate As String = "z-404932"

        'Create new array of valid file names from FileInfo
        Dim dir As New DirectoryInfo(HttpContext.Current.Request.ServerVariables("APPL_PHYSICAL_PATH").ToString() & "images\picts\")
        Dim files As FileInfo() = dir.GetFiles(smate & "*.jpg")
        Dim nLength As Integer = Len(smate)
        Dim newFiles As New ArrayList()
        For Each file As FileInfo In files
            If Mid(file.Name, nLength + 1, 1) = "-" Or Mid(file.Name, nLength + 1, 1) = "." Then
                newFiles.Add(file.Name)
            End If
        Next

        'now sort the arraylist
        newFiles.Sort()

        'now see if we have anything
        If newFiles.Count = 0 Then
            newFiles(0) = "no-photo.jpg"
        End If

        tstSlides.ImageUrl = "../images/picts/" & newFiles(0)
        SlideShowExtender1.SlideShowServiceMethod = "GetSlides()"

Also in the code behind is the webservice:

<System.Web.Services.WebMethod(), System.Web.Script.Services.ScriptMethod()> _
      Public Shared Function GetSlides() As AjaxControlToolkit.Slide()
        Dim imgSlide(3) As AjaxControlToolkit.Slide

        imgSlide(0) = New AjaxControlToolkit.Slide("../images/picts/z-404932.jpg", "Autumn", "Autumn Leaves")
        imgSlide(1) = New AjaxControlToolkit.Slide("../images/picts/z-404932-2.jpg", "Creek", "Creek")
        imgSlide(2) = New AjaxControlToolkit.Slide("../images/picts/z-404932-3.jpg", "Landscape", "Landscape")
        imgSlide(3) = New AjaxControlToolkit.Slide("../images/picts/z-404932-4.jpg", "Dock", "Dock")

        Return (imgSlide)

smate is hardcoded for testing, it will be a querystring.  Now this all works fine.  I have no problem setting tstSlides.ImageUrl or SlideShowExtender1.SlideShowServiceMethod in the code behind.  But now I want to pass in newFiles.  But to simplify things for now, I could get by with passing in a simpler variale, smate.  So I add the parameters as so:

SlideShowExtender1.SlideShowServiceMethod = "GetSlides(smate)"

and

<System.Web.Services.WebMethod(), System.Web.Script.Services.ScriptMethod()> _
      Public Shared Function GetSlides(ByVal smate As String) As AjaxControlToolkit.Slide()
        Dim imgSlide(3) As AjaxControlToolkit.Slide

        imgSlide(0) = New AjaxControlToolkit.Slide("../images/picts/z-404932.jpg", "Autumn", "Autumn Leaves")
        imgSlide(1) = New AjaxControlToolkit.Slide("../images/picts/z-404932-2.jpg", "Creek", "Creek")
        imgSlide(2) = New AjaxControlToolkit.Slide("../images/picts/z-404932-3.jpg", "Landscape", "Landscape")
        imgSlide(3) = New AjaxControlToolkit.Slide("../images/picts/z-404932-4.jpg", "Dock", "Dock")

        Return (imgSlide)

Note that I haven't changed anything in the webservice yet except for the new parameter.  The slideshow no longer works.  What am I not understanding?