Thanks Steve.

I copy the pictures onto new blank slides.
There is no need for the picture to be in a content placeholder.
There is a need to display the pictures in full size of slide (minus
some margin).

I went into the direction of chart content placeholder because when I
did manual trials, it produced the best results, unlike normal pasting
onto blank slide which produced inconsistent and unexpected results.

I always start with copy Excel range as picture (Shift+Edit), and
accept the defaults
(Appearance: As shown on screen, Format: Picture).

Paste
I tried four machines with same build of XP and Office 2003 sp2. On
two machines the output was OK, while on the other two, the picture
was cutoff. It happened even when I sized the xl range to the size of
the slide.This is very strange and I could not expect a solution for
it from a remote newsgroup member.
Pasting into chart content placeholder always produced good results,
and scaling was done automatically. This is the oly reason why I
explored that direction. However, you ruled out this option, because
there is no way to programmatically paste a picture into the
placeholder.

PasteSpecial (ppPasteEnhancedMetafile)
Using PasteSpecial solved the issue of truncated picture.
I used your tips to modify my code. I list below the relevant portions
of it, for the benefit of other users.
It now maximizes the picture without distortion, and centers it
horizontally.

I still have 2 puzzles:
1.  You had:
   Dim oSh As Shape
   Set oSh = ActivePresentation.Slides(3).Shapes.Paste(1)
And it works
In my implementation, I had a Type Mismatch error, and I had to leave
oShape undefined. Why?
2.  The placement of the "Copy" statement turned out to be critical.
It works ok where it is placed, but when it is moved to the line:
'**Bad Location, the picture was truncated.
It appears that picture size was different, depending on where it was
copied. Why?

Sub PP_Creator()
  Dim oPPApp As PowerPoint.Application
  Dim oPres As PowerPoint.Presentation
  Dim oSlide As PowerPoint.Slide
  Dim oShape  'As ???
  Dim strPP_FileName As String
  Dim intSlideWidth As Integer
  Dim intSlideHeight As Integer

  intSlideWidth = 756     '** = 10.5 Inches
  intSlideHeight = 576    '** =  8.0 Inches
  strPP_FileName = "C:\MyFolder\MyFileName.ppt"

  '** Bad Location
'   Worksheets(1).Range("ppExport").CopyPicture Appearance:=xlScreen,
Format:=xlPicture

  Set oPPApp = New PowerPoint.Application
  Set oPres = oPPApp.Presentations.Add(True)
  Set oSlide = oPres.Slides.Add(1, ppLayoutBlank)

  With oPres.PageSetup
     .SlideSize = ppSlideSizeCustom
     .SlideWidth = intSlideWidth
     .SlideHeight = intSlideHeight
     .SlideOrientation = msoOrientationHorizontal
  End With

  '** Good Location
  Worksheets(1).Range("ppExport").CopyPicture Appearance:=xlScreen,
Format:=xlPicture

  oSlide.Shapes.PasteSpecial (ppPasteEnhancedMetafile)
  Set oShape = oSlide.Shapes(1)

  With oShape
     .LockAspectRatio = True

     '** Resize picture: Test if Width or Height is real constraint.
     If .Width / intSlideWidth > .Height / intSlideHeight Then
        '** It is a Width constraint
        .Width = oPres.PageSetup.SlideWidth - 20
     Else
        '** It is a Height constraint
        .Height = oPres.PageSetup.SlideHeight - 30
     End If

     .Top = 20
     .Left = (intSlideWidth - .Width) / 2
  End With

  oPres.SaveAs strPP_FileName

  Set oSlide = Nothing
  Set oPres = Nothing
  Set oShape = Nothing
End Sub

Thanks
dR