First off, this is aweful for SEO.
You could go over to ninjoomla.com and hit File>"Save Page" in your browser, then edit that for your site. The key part is in the head:
<meta http-equiv="refresh" content="10;url=http://ninjaforge.com/">
The "10" is the time in seconds before the user gets redirected to the url following that (You would change that to
yourdomain.com/index.php
CG Icon, that IS tricky
... maybe possible with cookies... but the splash screen would appear when using a different PC or after the browser cache is cleared.