Essentially there are no blank slides, rather you create a gap into which you insert text using the text tool button.  So if you wanted to precede your photo slide show with an introductory area of text, just push your photos to the right, then click on the text button and type in your text.  This will appear on one of the tracks below the photo track.  You can then adjust the timing visually, that is, you can show your text and have it transition into your first photo by moving the text block around relative to the first photo.