The following will replace a selected inline image with the named image and set the image width to the same size as the original, which for similar aspect ratio pictures means the same size:
Code:
Dim oRng As Range
Dim oShp As InlineShape
Dim lngWidth As Long
Set oRng = Selection.InlineShapes(1).Range
lngWidth = Selection.InlineShapes(1).Width
oRng.Text = ""
Set oShp = oRng.InlineShapes.AddPicture("C:\Path\Filename.jpg")
oShp.LockAspectRatio = msoTrue
oShp.Width = lngWidth