In order to do this, you need to be able to supply some condition that will identify your shapes uniquely; they might be the only picture shapes on a slide, the only shapes on an otherwise blank slide, or any shapes added after you've first gotten a count of the number of shapes on the slide. Once you have a condition you can apply this can build an array of the shapes that meet the condition then group them:
Sub GroupCertainShapes()
Dim x As Long
Dim sTemp As String
Dim aShapeList() As String
Dim lShapeCount As Long
With ActivePresentation.Slides(1)
' iterate through all shapes on the slide
' to get a count of shapes that meet our condition
For x = 1 To .Shapes.Count
' Does the shape meet our condition? count it.
If .Shapes(x).Type = msoAutoShape Then
lShapeCount = lShapeCount + 1
End If
Next
' now we know how many elements to include in our array,
' so redim it:
ReDim aShapeList(1 To lShapeCount)
' Reset the shape counter
lShapeCount = 0
' Now add the shapes that meet our condition
' to the array:
For x = 1 To .Shapes.Count
' apply some criterion for including the shape or not
If .Shapes(x).Type = msoAutoShape Then
lShapeCount = lShapeCount + 1
aShapeList(lShapeCount) = .Shapes(x).Name
End If
Next
' and finally form a group from the shapes in the array:
If UBound(aShapeList) > 0 Then
.Shapes.Range(aShapeList).Group
End If
End With
End Sub