So far as I know you can't change the source of a picture, you need to delete the old one and insert a new one
Here's a start
strPic ="Picture Name"
Set shp = ws.Shapes(strPic)
'Capture properties of exisitng picture such as location and size
With shp
t = .Top
l = .Left
h = .Height
w = .Width
End With
ws.Shapes(strPic).Delete
Set shp = ws.Shapes.AddPicture("Y:ourPicturePathAndFile.Name", msoFalse, msoTrue, l, t, w, h)
shp.Name = strPic
shp.ScaleHeight Factor:=1, RelativeToOriginalSize:=msoTrue
shp.ScaleWidth Factor:=1, RelativeToOriginalSize:=msoTrue
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