In Gridview Set Image Column programatically

[EdwardH]

In Gridview I have column representing Thumbnail image which works fine.:
<asp:ImageField DataImageUrlField="PicFile1"
DataImageUrlFormatString="Images/A009/Thumbs/{0}">
</asp:ImageField>

I want the fixed 'A009' to be replaced by Session variable - "ClCode"

VB in code-behind I have:
iPicFile = ImageString & Session("ClCode") & "/Thumbs/{0}"
How can "iPicFile" variable be used in DataImageUrlFormatString?
With thanks