All the examples I've seen of calling a URL field to be displayed as an image have been through C# code. Which is quite annoying since most of web part bits can be written without using C# code. You might also need to go down that road here... perhaps a custom field control or user control.
Even if your code above did get the value for HeaderImageUrl, it wouldn't work as Url values are (confusingly):
[the actual url specified][some wierd delimeter string][the description field of the url (usually the actual url again if your don't specify it)]
So you would need to split the string with te delimeter values and use the [0] (the first) as what you inject into your image url.
Please do post if you find a way to do this without writing C# code.
p.s. Have you considered using a DataFormWebPart which has a xsl section in which you might be able to do extractly what you need?!
Example: You can stick the following syntax into any aspx / ascx file:
<%@ Register tagprefix="SharePoint" namespace="Microsoft.SharePoint.WebControls" assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register tagprefix="WebPartPages" namespace="Microsoft.SharePoint.WebPartPages" assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="PublishingWebControls" Namespace="Microsoft.SharePoint.Publishing.WebControls" Assembly="Microsoft.SharePoint.Publishing, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<WebPartPages:DataFormWebPart runat="server" IsIncluded="True" FrameType="None"
NoDefaultStyle="TRUE" ViewFlag="0" Title="My List Display"
Default="FALSE" ID="dfwp_MyListDisplay"
DisplayName="My List Display">
<DataSources>
<SharePoint:SPDataSource runat="server" DataSourceMode="List" UseInternalName="true"
selectcommand='<View><Query><Where><Eq><FieldRef Name="Category"/><Value Type="Text">Header</Value></Eq></Where></Query></View>' id="Utility_x0020_Navigation1"><SelectParameters><WebPartPages:DataFormParameter Name="ListName" ParameterKey="ListName" PropertyName="ParameterValues" DefaultValue="My List Display"/>
<asp:Parameter Name="ListName" DefaultValue="LIST NAME" />
<asp:Parameter Name="WebUrl" DefaultValue="<% $SPUrl:~SiteCollection/ %>" />
</SelectParameters>
</SharePoint:SPDataSource>
</DataSources>
<ParameterBindings>
<ParameterBinding Name="dvt_apos" Location="Postback;Connection"/>
<ParameterBinding Name="PageURL" Location="ServerVariable(URL)" DefaultValue=""/>
<ParameterBinding Name="SiteUrl" Location="ServerVariable(SERVER_NAME)" DefaultValue=""/>
</ParameterBindings>
<datafields>@Title,Title;@URL,URL;@Category,Category;@Parent_x0020_item,Parent_x0020_item;@Sort_x0020_Order,Sort Order;@ID,ID;@ContentType,Content Type;@Modified,Modified;@Created,Created;@Author,Created By;@Editor,Modified By;@_UIVersionString,Version;@Attachments,Attachments;@File_x0020_Type,File Type;@FileLeafRef,Name (for use in forms);@FileDirRef,Path;@FSObjType,Item Type;@_HasCopyDestinations,Has Copy Destinations;@_CopySource,Copy Source;@ContentTypeId,Content Type ID;@_ModerationStatus,Approval Status;@_UIVersion,UI Version;@Created_x0020_Date,Created;@FileRef,URL Path;</datafields>
<xsl>
YOUR XSLT HERE
</xsl>
</WebPartPages:DataFormWebPart>