Open up your phpnuke config.php file and look for this...

$AllowableHTML = array("b"=>1,
"i"=>1,
"a"=>2,
"em"=>1,
"br"=>1,
"strong"=>1,
"blockquote"=>1,
"tt"=>1,
"li"=>1,
"ol"=>1,
"ul"=>1);

Add or change it looks like the following...

$AllowableHTML = array("b"=>1,
"strong"=>1,
"em"=>1,
"i"=>1,
"u"=>1,
"br"=>1,
"p"=>2,
"a"=>2,
"h1"=>1,
"h2"=>1,
"h3"=>1,
"h4"=>1,
"h5"=>1,
"h6"=>1,
"center"=>1,
img"=>2,
"alt"=>1,
"table"=>2,
"tr"=>2,
"td"=>2,
"div"=>2,
"font"=>2,
"style"=>2,
"blockquote"=>1,
"tt"=>1,
"li"=>1,
"ol"=>1,
"ul"=>1);