From: http://communityserver.org/forums/thread/557086.aspx
Find more in: http://dbvt.com/search/SearchResults.aspx? Q = export image. aspx & s = 5
Download the patch at: http://dbvt.com/files/folders/addons/entry5198.aspx
1) Drop the daveburke. cs21.captcha. DLL into your \ bin directory
2) copy your image. aspx into your web \ blogs folder.
3) Now you'll need to update your comment form and your contact form. complete samples located in \ samples folder. do not copy the sample files to your site, since they have been customized for my installation and will most likely conflict with your blog's theme.
Comment Form
3A) Open your theme's SKIN-ENTRYVIEWCONTAINER.ASCX file and add the following register statement and replace the <blog: commentform/> with the line listed below.
<% @ Register tagprefix = "dbvt" namespace = "daveburke. cs21.captcha" assembly = "daveburke. cs21.captcha" %>
<Dbvt: commentform runat = "server" id = "form"/>
3b) Open SKIN-COMMENTFORM.ASCX and add the following HTML between the tbcomment textbox and the chkremember checkbox. Feel free to reformat to match your comment form style.
<DT>
<DD>
<Asp: requiredfieldvalidator id = "reqcaptcha" runat = "server" display = "dynamic"
Errormessage = "Please enter the code" controltovalidate = "codenumbertextbox"> </ASP: requiredfieldvalidator>
<Asp: Label runat = "server" id = "lblimage" font-bold = "true" forecolor = "red"/>
<P>
</P>
</DD>
<DD>
& nbsp;
</DD>
</DT>
<DT> enter code here: <em> required </em>
<DD>
<Asp: textbox id = "codenumbertextbox" runat = "server" cssclass = "codebox"/>
</DD>
</DT>
[From alphawu. Blog]