I'm not familiar with Yahoo Sitebuilder, but I think I know what to do.
I'm assuming that the images, background, etc. are all local files (i.e. they reside on your hard drive). When you e-mailed your friend the page all you attached was the actual HTML document. In order for your friend to view the images and other media you use in your pages (music, backgrounds stylesheets, scripts, etc.) he has to have access to them. This means you need to attach every media file you use to the e-mail in addition to the HTML document.
The simplest way I can think of would be to open the page in question in Internet Explorer and click File-->Save As to open the "Save Web page" dialogue. Change the "Save As Type" field to "Web Page, Complete (*.htm,*.html)", enter a file name and click the "Save" button.
This will save the web page and media files. The media files will be saved in a folder called "Your Page_files" where "Your Page" is the name you saved the web page as. Make a ZIP file of the web page and the folder and e-mail that to your friend.
Edited by Amazing Andrew, 12 January 2006 - 06:45 PM.