ALT tag in featured image

Hi there,
I’m here again to try to resolve this “problem”.
In the past I asked your support for the same question, and it looks I was able to fix it, but now I realize that I did not.
Please may you help me? I spended all the day trying to insert the ALT tag in post featured images, but I’m don’t know php language and I failed 1000 times
In my old post you said:
Yes, you need to edit your index.php and single.php templates, then find all occurences of “wp_get_attachment_image_src”
Then add the params for the alt tabs as described here:

Then add the alt attribute to the image below.
I tried to edit the files index.php and single.php, but no success…
Below is just a part of the code that I need to edit… How to add the params for the alt tab?

<?php // IF Post type is Standard (false) 	
			if(function_exists( 'get_post_format' ) && get_post_format( $post->ID ) != 'gallery' && get_post_format( $post->ID ) != 'video' && has_post_thumbnail()) { 
					$thumbbig = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'portfolio-full' );
					$attachment_image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'portfolio-main');
		?>
						<div class="pic">
							<a href="<?php echo $thumbbig[0];?>" rel="prettyPhoto" title="<?php echo $post->post_title; ?>">
								<img src="<?php echo $attachment_image[0];?>" /><div class="img_overlay_zoom"></div>
							</a>
						</div>
						
						
						<div class="h20"></div>	
		
		<?php } // IF Post type is Standard :: END ?>
		
		<?php // IF Post type is Gallery
		if (( function_exists( 'get_post_format' ) && get_post_format( $post->ID ) == 'gallery' )) {
					$args = array(
                            'numberposts' => -1, // Using -1 loads all posts
                            'orderby' => 'menu_order', // This ensures images are in the order set in the page media manager
                            'order'=> 'ASC',
                            'post_mime_type' => 'image', // Make sure it doesn't pull other resources, like videos
                            'post_parent' => $post->ID, // Important part - ensures the associated images are loaded
                            'post_status' => null,
                            'post_type' => 'attachment'
                            );

					$images = get_children( $args );
					
					if($images){ ?>
					
					<div class="flexslider">
				        <ul class="slides">
						<?php foreach($images as $image){ 
								$attachment = wp_get_attachment_image_src($image->ID, 'portfolio-full');
								$thumb = wp_get_attachment_image_src($image->ID, 'portfolio-main'); 
						?>
								<li class="pic">
										<a href="<?php echo $attachment[0] ?>" rel="prettyPhoto[gallery]" title="<?php echo $image->post_title; ?>" >
											<img src="<?php echo $thumb[0] ?>" /><div class="img_overlay_zoom"></div>
										</a>
								</li>
						<?php } ?>						
						</ul>  
					</div>

I hope you can help me, alt tag is important for SEO…
Ciao.
Enrico