I'd go download the RnR Starter Set add-in. It includes a "place exactly"
tool that will do exactly what you want.

Set up one pix the way (size and position) you want it, then click on the
icon to pick up its settings. Then use the hammer to "hammer" the other pix
into place. One click's all it takes.

Starter Set is free. Here. http://www.rdpslides.com/pptools/

What Echo said.

And here's a recipe for getting the job done with the StarterSet:

TIP: Make several images the same size
http://www.rdpslides.com/pptools/starterset/FAQ00034.htm

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
================================================

And here's a way without any hammers at all!

Resize one pic, select it and then run this macro

Sub picsize()
Dim postop As Integer
Dim posleft As Integer
Dim sizewidth As Integer
Dim sizeheight As Integer
Dim oSld As Slide
Dim oShp As Shape
postop = ActiveWindow.Selection.ShapeRange.Top
posleft = ActiveWindow.Selection.ShapeRange.Left
sizewidth = ActiveWindow.Selection.ShapeRange.Width
sizeheight = ActiveWindow.Selection.ShapeRange.Height
For Each oSld In ActivePresentation.Slides
For Each oShp In oSld.Shapes
If oShp.Fill.Type = msoFillPicture Then
With oShp
.Width = sizewidth
.Height = sizeheight
.Top = postop
.Left = posleft
End With
End If
Next oShp
Next oSld
End Sub