Lana,
Do your pictures have the same dimensions, and are they perfectly stacked
upon eachother you could do this.

For the first picture set an entrance animation with previous, for the
second picture set an entrance animation after previous with a delay of 5
seconds. You can set the delay in the timings tab of the dialog. Repeat that
process for the third picture. Set the slide transition time to 15 seconds.
If your pictures do not have the same dimensions. For the first picture set
the entrance animation with previous, select the picture again and set an
exit animation after previous, with a delay of 5 seconds. For the second
picture, set the entrance effect to after previous without a delay, select
again and apply exit effect with a delay of 5 seconds, for the third apply
an entrance effect without a delay. Set the slide transition time to 15
seconds.