Generate thumb image function

hi,
I need to generate thumbnail for image dynamically and use it in this style
<img src='<% createthumbnail(Eval("Imageurk") %>' />
I could use these codes to generate thumbnail.
imageUrl = "/images/St/Sample.png"  
    Dim fullSizeImg as System.Drawing.Image
    fullSizeImg = System.Drawing.Image.FromFile(Server.MapPath(imageUrl))
    Response.ContentType = "image/gif"
    fullSizeImg.Save(Response.OutputStream, ImageFormat.Gif)
 
Now how could I declare function that get image url and return thumbnail for input image to use it for above style.
 
 
 
 
 
 
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