a me non risulta. Ho fatto una prova e l'immagine viene sempre mostrata nella sua grandezza originale. Ecco la pagina di prova:
codice:
<%@ Page Language="VB" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
        If Not Me.IsPostBack Then
            Me.ListBox1.Items.Add(New ListItem("prova.jpg", "http://localhost/prove_nicon/prova.jpg"))
            Me.ListBox1.Items.Add(New ListItem("DSCN0004.jpg", "http://localhost/prove_nicon/DSCN0004.JPG"))
            Me.ListBox1.Items.Add(New ListItem("DSCN0005.jpg", "http://localhost/prove_nicon/DSCN0005.jpg"))
            Me.ListBox1.Items.Add(New ListItem("DSCN0006.jpg", "http://localhost/prove_nicon/DSCN0006.jpg"))
            Me.ListBox1.SelectedIndex = 0
            
            Me.Preview_Image.ImageUrl = Me.ListBox1.SelectedValue
            
        End If
        
    End Sub

    Protected Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        Me.Preview_Image.ImageUrl = Me.ListBox1.SelectedValue
    End Sub
</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Pagina senza titolo</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ListBox ID="ListBox1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged"></asp:ListBox>
    </div>
        <asp:Image ID="Preview_Image" runat="server" />
    </form>
</body>
</html>