mojoPortal content management system has a built in feature to show a friendly page if the visitor tries to visit an invalid url in your site.
By default it uses the PageNotFound.aspx page in the root of the site, but if you wanted to you could copy that file and rename it and customize it with additional text and or graphics. So for example you could copy it and rename it to 404.aspx
Then you can specify to use your custom version by adding this setting in your user.config file:
<add key="Custom404Page" value="/404.aspx" />
Note that if you do not want the default error message to display, just add Visible="false" on the:
<asp:Literal ID="litErrorMessage" runat="server" EnableViewState="false" />
in your copy of the the page.