In Office 2010 Images will usually go into empty content placeholders all by themselves.

Sub AddPic()
Dim osld As Slide
Set osld = ActivePresentation.Slides(1) ' change number to suit
'obviously adjust path to suit your image
'Left and Top will be ignored but must have a value
osld.Shapes.AddPicture "C:\Users\John\Desktop\Pic1.jpg", _
LinkToFile:=msoFalse, _
SaveWithDocument:=msoTrue, _
Left:=0, _
Top:=0
End Sub
Note in 2007 this does not work.