My custom error page works fine, however I want to add graphics so the page will look like the rest of my site. When I do so, the images come up as not found (x in a circle). The code is OK, the image links are to the same folder as the images on my index page which work fine.

Also, how do I set up custom error pages for add-on domains?

Please tell me a link to your error page
Also you can't add an error page to addon domain from control panel. Just provide us with a needful info and we'll do it ourselves.

How can i set error pages common to all the new accounts in my reseller plan ?
(same to cpanel skeleton for the index.html....)

thank you

There is no way to to that.