The images in the slideshow come from the featured images on the individual pages. You can add as many images as pages that you have on the site. First add the image to the page and then tell the slideshow where to find the image, update the page.

To tell the slideshow, From the dashboard view, select Theme Options under the Appearance tab.

From the theme options, select Front Page Settings

Pages listed on left already appear on slideshow, list on right are the pages to select from, click the plus sign to add more pages to the slideshow, then select update options.