There is no speical coding required to do this, if you are already reading the images from a folder. It will automatically get updated.
If you have a slideshowextender that has the SlideShowService method set to one of the webmethods, like following, then you need to ensure that you are reading the images from the very same folder and building the slides.
<ajaxToolkit:SlideShowExtender ID="SlideShowExtender1" runat="server" TargetControlID="Image1" SlideShowServiceMethod="GetSlidesFromFolder" />
And perhaps, you have the GetSlidesFromFolder definition as follows:
public static AjaxControlToolkit.Slide GetSlidesFromFolder()
//Do a server.mappath to the images folder and find out how many images you have.
string images = System.IO.Directory.GetFiles(Server.MapPath("Images"));
//Based on the image count, create the slide array.
AjaxControlToolkit.Slide slides = new AjaxControlToolkit.Slide[images.length];
//Loop thru the images and create slides
for (int counter = 0; counter < images.Length; counter++)
//Use System.IO.File.GetFilename to get the file name.
slides[counter] = new AjaxControlToolkit.Slide("images/" + System.IO.File.GetFileName(file));
//return the images as slides.
Hope this helps.