First, click Slide Show > Custom Animation.
Select all four pictures.
Click Add Effect > Entrance > Appear/Fade.
Select the four added effects.
Set the added effects to have Start: "After Previous" instead of On Click.
Now, right click on the effects, select Timing.
Set the delay to 1 or 2 seconds. (For PowerPoint 2000 and below, you can set
the "Start Animation" under Order & Timing tab)
Click ok to apply the changes.
Holler back if you need more help.