The control above is initialized with this code. The italic properties are optional:

<ajaxToolkit:SlideShowExtender ID="SlideShowExtender1" runat="server" 
                                                    TargetControlID="Image1" 
                                                    SlideShowServiceMethod="GetSlides" 
                                                    AutoPlay="true" 
                                                    ImageTitleLabelID="imageTitle"
                                                    ImageDescriptionLabelID="imageDescription"
                                                    NextButtonID="nextButton" 
                                                    PlayButtonText="Play" 
                                                    StopButtonText="Stop" 
                                                    PreviousButtonID="prevButton" 
                                                    PlayButtonID="playButton" 
                                                    Loop="true" /> 

• SlideShowServicePath - Path to the webservice that the extender will pull the images from.
• SlideShowServiceMethod - The webservice method that will be called to supply images. The signature of the method must match this:

                                                          [System.Web.Services.WebMethod]
                                                          [System.Web.Script.Services.ScriptMethod]
                                                          public AjaxControlToolkit.Slide[] GetSlides() { ... }
                                                      

  Note that you can replace "GetSlides" with a name of your choice, but the return type and parameter name and type must exactly match, including case.
• ContextKey - User/page specific context provided to an optional overload of the web method described by ServiceMethod/ServicePath. If the context key is used, it should have the same signature with an additional parameter named contextKey of type string:

                                                      [System.Web.Services.WebMethod]
                                                      [System.Web.Script.Services.ScriptMethod]
                                                      public AjaxControlToolkit.Slide[] GetSlides(string contextKey) { ... }

  Note that you can replace "GetSlides" with a name of your choice, but the return type and parameter name and type must exactly match, including case.
• UseContextKey - Whether or not the ContextKey property should be used. This will be automatically enabled if the ContextKey property is ever set (on either the client or the server). If the context key is used, it should have the same signature with an additional parameter named contextKey of type string (as described above).
• NextButtonID - ID of the button that will allow you to see the next picture.
• PlayButtonID - ID of the button that will allow you to play/stop the slideshow.
• PreviousButtonID - ID of the button that will allow you to see the previous picture.
• PlayButtonText - The text to be shown in the play button to play the slideshow.
• StopButtonText - The text to be shown in the play button to stop the slideshow.
• PlayInterval - Interval in milliseconds between slide transitions in play mode.
• ImageTitleLabelID - ID of Label displaying current picture's title.
• ImageDescriptionLabelID - ID of Label describing current picture.
• Loop - Setting this to true will allow you to view images in a round-robin fashion.
• AutoPlay - Setting this to true will play the slideshow automatically on render.