Usually the reason for this issue is that you don't access Sitebuilder by your domain name. Please check this link on how to access your Sitebuilder correctly:

http://kb.siteground.com/article/How_to_log_in_SiteBuilder.html<

Such a problem also occurs when you have HotLink protection enabled. To resolve this try disabling the HotLink protection from cPanel.

Finally, it is not recommended to have many images on your Sitebuilder pages. This is because Sitebuilder stores most of its information in a mysql database. Loading images from there may cause high server load and time outs.