how to Including thumbnails in my home theme

 if (function_exists ('add_theme_support')) {
add_theme_support ('post-thumbnails');
set_post_thumbnail_size (150, 150, true);
add_image_size ('featured', 990, 335, true);
 }

 <? Php get_header ();?>


<div id="left">
<? Php $ categories = get_option_tree ('blogcategory','');?>
        <? Php query_posts ($ query_string. '& Cat ='. $ Categories.'');?>
<? Php if (have_posts ()): while (have_posts ()): the_post ();?>
    <Div class = "post" id = "post-<? Php the_ID ();?>" <? Php post_class ();? >>
        <div class="posttitle">
            <div class="commentbox"> <? php if (comments_open ()) {comments_popup_link ('0 ', '1', '%');} else {echo "-";}?> </ div>
                <h3> <a href = "<? php the_permalink ()?>" rel = "bookmark"> <? php the_title ();?> </ a> </ h3>
                <span class="postmeta"> <div class="authorlink"> <? php the_author_posts_link ();?> </ span> <span class="date"> <? php the_time ('d M Y');? > </ span> <span class="tags"> <? php if (get_option_tree ('tagcat', 'Categories') == "Categories") {?> <? php the_category (',')?> <? php} else {?> <? php the_tags ('', ',','');?> <? php}?> </ span> </ div>
       </ Div>
          <div class="posttext">
            <? Php if (get_option_tree ('postview', 'Full Posts ") ==" Full Posts ") {?>
<? Php the_content ('Continue Reading →');?>
            <? Php} else {?>
                <? Php the_excerpt ('Continue Reading →');?>
            <? Php}?>
            </ Div>
            <div class="postend">
            <div class="readmorediv"> <a href = "<? php the_permalink ()?>"> read more </ a> </ div>
            </ Div>
        </ Div>
        <? Php endwhile;?>
   <? Php else:?>
    <? Php endif;?>
</ Div>
<? Php get_sidebar ();?>
<div id="pagination">
<? Php if (function_exists ('wp_pagenavi')) {wp_pagenavi ();}?>
    <div class="totop">
    <a href="#top"> Go To Top </ a>
    </ Div>
</ Div>
<? Php get_footer ();?>
[/ PHP]


And maybe served excerpts:


[PHP] <? Php
featured_excerpt_length function ($ length) {
16 return;
}
wpe_excerptmore function ($ more) {
    return '...';
}
wpe_excerpt function ($ length_callback ='', $ more_callback ='') {
    global $ post;
    if (function_exists ($ length_callback)) {
        add_filter ('excerpt_length', $ length_callback);
    }
    if (function_exists ($ more_callback)) {
        add_filter ('excerpt_more', $ more_callback);
    }
    $ Output = get_the_excerpt ();
    $ Output = apply_filters ('wptexturize', $ output);
    $ Output = apply_filters ('convert_chars', $ output);
    echo $ output;
}
ShortenText function ($ text)
{
$ Chars_limit = 22;
$ Chars_text = strlen ($ text);
$ Text = $ text. "";
$ Text = substr ($ text, 0, $ chars_limit);
$ Text = substr ($ text, 0, strrpos ($ text, ''));


if ($ chars_text> $ chars_limit) {
$ Text = $ text. "...";
}
return $ text;
}
 ?>