I've went with a Macro solution, although the script may not be correct (I've just read a little about VBA yesterday). I couldn't find a style constant, so I've added the properties that interested me (a shadow).
Public Sub AddShadow()
Dim document As document
Dim shape As InlineShape
If Application.ActiveInspector Is Nothing Then
Exit Sub
End If
If Not (TypeOf Application.ActiveInspector.CurrentItem Is MailItem) Then
Exit Sub
End If
Set document = Application.ActiveInspector.CurrentItem.GetInspector.WordEditor
For Each shape In document.InlineShapes
With shape
If .Type = wdInlineShapePicture And .Width > 70 And .Height > 70 Then
With .Shadow
.Visible = msoTrue
.Style = msoShadowStyleOuterShadow
.Blur = 30
.Transparency = 0.29
End With
End If
End With
Next
End
End Sub