Ti ho detto che funzionaaaaaaa!

codice:
        <asp:GridView ID="GridView1" runat="server" EnableViewState="false" AutoGenerateColumns="true" AutoGenerateDeleteButton="true" OnRowDataBound="GridView1_RowDataBound" OnRowDeleting="GridView1_RowDeleting">
            <Columns>
                <asp:CommandField ButtonType="Image" ShowDeleteButton="true" DeleteImageUrl="../../../../immagini/cancella.gif" />
                
                
                <%-- 
                <asp:CommandField ButtonType="Image" 
                       ShowEditButton="True"  
                       ShowDeleteButton="True"
                       ItemStyle-HorizontalAlign="Center" 
                       HeaderText="OPZIONI" 
                       EditImageUrl="~/IMG/edit.png"
                       DeleteImageUrl="~/IMG/delete.png" 
                       CancelImageUrl="~/IMG/annulla.png" 
                       UpdateImageUrl="~/IMG/aggiorna.png"  />
                --%>
            </Columns>
        </asp:GridView>

Dim imgb As ImageButton = DirectCast(e.Row.Cells(1).Controls(0), ImageButton)
imgb.Attributes.Add("onclick", "if (!confirm('Sei sicuro di voler cancellare questo record?') ) {return false;}")