You can automatically add a thumbnail image of your image at the same time you save your image.  To save your image, use the Salva immagine corrente con nome... / Save current image as... command (File / File menu).  This command runs the Salva immagine con nome / Save Image As dialog box.  You use the controls under the Indice anteprime / Thumbnail index heading on this dialog box to create and update thumbnail index web pages.

You’ll need to create a new thumbnail index before you can insert the first thumbnail.  Press the Crea nuovo indice anteprime... / Create new thumbnail index... button to run the Proprietà indice anteprime / Thumbnail index properties dialog box.  You’ll select a thumbnail index template and make choices about the size and style for the thumbnails. If you’d like to see what the thumbnail index will look like, press the Anteprima / Preview button to display a sample web page in your computer’s web browser.