function preg_img($img) {
    return preg_replace( '/(<img.*?)(style=.+?[\'|"])|((width)=[\'"]+[0-9]+[\'"]+)|((height)=[\'"]+[0-9]+[\'"]+)/i', '$1' , $img); 
}
echo preg_img('<img src="1.png" width="500" height="300" style="float:left"');

上面的代码连iframe都会清除，如果只想清除img的话，可以使用下面这个

function preg_img($str){
    //去掉图片宽度
        $search = '/(<img.*?)width=(["\'])?.*?(?(2)\2|\s)([^>]+>).*?>/is';
        //去掉图片高度
        $search1 = '/(<img.*?)height=(["\'])?.*?(?(2)\2|\s)([^>]+>).*?>/is';
        $search2 = '/(<img.*?)style=(["\'])?.*?(?(2)\2|\s)([^>]+>).*?>/is';
        $content = preg_replace($search,'$1$3',$str);
        $content = preg_replace($search1,'$1$3',$content);
        $content = preg_replace($search2,'$1$3',$content);
    return $content;
}