E' passato molto tempo.
il controllo è WebControlCaptcha.dll ma proprio non ricordo dove l'ho pescato, sicuramente in rete.

ho ripescato La direttiva per il controllo:
<%@ Register Assembly="GURU4.net.Web.Controls.CaptchaLibrary" Namespace="GURU4.net.Web.Controls.CaptchaLibrary" TagPrefix="ccl" %>

ed il form:

codice:
    <form id="frmMain" runat="server">
    <div class="content">
        
        <h1>Titolo</h1>
        
        <asp:PlaceHolder ID="phResult" runat="server" Visible="false">
            <div class="response">
                

<asp:Label ID="lblName" runat="server" /></p>
                

Close window</p>
            </div>
        </asp:PlaceHolder>
    
        <asp:PlaceHolder ID="phCaptcha" runat="server">
            
            <hr />
            
            <h2>Please enter your email address and the message</h2>
            
            <div class="fieldset">
                <div class="fieldvalidator">
                    <asp:RegularExpressionValidator id="fvFirstname1" runat="server" ErrorMessage="Insert correct e-mail" ControlToValidate="da" ValidationExpression="\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"></asp:RegularExpressionValidator>
                    <asp:RequiredFieldValidator ID="fvFirstname2" ErrorMessage="Insert your e-mail address" ControlToValidate="da" runat="server" ForeColor="" SetFocusOnError="true" Display="Dynamic" />
                </div>
                <label for="da">e-mail address:</label>
                <asp:TextBox ID="da" runat="server" Width = "400px"></asp:TextBox>
            </div>
            
            <div class="fieldset">
                <div class="fieldvalidator">
                    <asp:RequiredFieldValidator ID="fvLastname1" ErrorMessage="Please, insert message" ControlToValidate="messaggio" runat="server" ForeColor="" SetFocusOnError="true" Display="Dynamic" />
                </div>
                <label for="messaggio">Text:</label>
                <asp:TextBox ID="messaggio" runat="server" Height="100px" Style="position: static" Width="400px"></asp:TextBox>
                <%--<asp:TextBox ID="messaggio" runat="server"></asp:TextBox>--%>
            </div>
            
            <hr />
            
            <h2>Please enter the characters shown in the image (6 characters)</h2>
            

</p>
            
            <div class="fieldset">
                <label>Image:</label>
                <div class="captcha"><ccl:VisualCaptcha ID="vcCaptcha" runat="server" Width="260px" Height="80px" ChallengeTextLength="6" ToolTip="Codice di verifica" IgnoreCase="true" /></div>
            </div>
            
            <div class="fieldset">
                <div class="fieldvalidator">
                    <asp:RequiredFieldValidator ID="fvCaptcha1" ErrorMessage="Type the characters that you see in the image" ControlToValidate="txtCaptcha" runat="server" ForeColor="" SetFocusOnError="true" Display="Dynamic" />
                    <ccl:VisualCaptchaValidator ID="fvCaptcha2" runat="server" ErrorMessage="The characters typed do not match the image displayed" ControlToValidate="txtCaptcha" ForeColor="" SetFocusOnError="true" AssociatedVisualCaptchaControlId="vcCaptcha" Display="Dynamic" />
                </div>
                <label for="txtCaptcha">Enter characters shown:</label>
                <asp:TextBox ID="txtCaptcha" runat="server"></asp:TextBox>
                <div class="fieldnote">If you can not read the characters in the image try to <asp:LinkButton ID="btnCaptchaRefresh" runat="server" Text="refresh the image" CausesValidation="false" OnClick="btnCaptchaRefresh_Click" /></div>
            </div>
            
            <hr />
            
            <div class="buttonset">
                <asp:Button ID="btnConfirm" runat="server" Text="Send message" CssClass="btn" OnClick="btnConfirm_Click" />
            </div>

        </asp:PlaceHolder>
    </div>
    
    </form>
Altro non mi ricordo anche perchè è da parecchio che non uso più asp.net.
Se non trovi il controllo te lo posto in privato
Buon lavoro.