The reason for this could be any number of things. Look at the HTML source generated by the script, and make sure that the files listed as src attributes actually exist and are readable by the server; also make sure that the directory where these files live is accessible to the server.

It is probably simplest to troubleshoot this particular problem with static HTML (e.g. write an HTML file whose content is one table with just one row on it). Once you get the static HTML to work, worry about generating it programmatically with a CGI script.