You apply an animation entrance to each picture. Align on pictures on the
slide. Right click on one and choose 'Custom Animation'. Now select all in
the order you want the to appear (click on the 1st, then with the shift key
down click the others). Choose the ENTRANCE >> FADE animation. While all are
still selected click on the START drop down and change to ON-CLICK. Now in
slideshow each picture will fade in at each click.
Hope that is helpful.