Esta pequena macro fará o trabalho para você:
Sub AddPictureBorders()
' Before running this, apply the style you want to one picture
' then use the formatting paintbrush to pick up the formatting
' This will apply the formatting to each picture.
Dim oSh As Shape
Dim oSl As Slide
For Each oSl In ActivePresentation.Slides
For Each oSh In oSl.Shapes
With oSh
' ignoring linked pictures, OLE objects
' and such
If .Type = msoPicture Then
.Apply
End If
End With
Next
Next
End Sub