Yes, On your code when you eval CustomerId or bind this Will return a value that will be the name of the image, with the Javascript that I send you you can say that if the return is null or blank or the condition that you wnat then return the default image name if not return the value that get the customer photo.I modify the code because I made a mistake. Will be like this
<script language="vbscript" runat="server">
Function EvalPhoto(ByVal Customer)
Dim Cid, ReturnImg
Cid = Customer
'If The Customer Exist or the condition that you want
If Cid is dbnull.value Then
ReturnImg = Cid
Else
ReturnImg = "Default Image"
End If
Return ReturnImg
End Function
</script>