If your current code manages all blocks with images replacing usage wp_get_attachment_image_src with output of hardcoded img tag, with echo wp_get_attachment_image() which will return img tag as string with default alt of image or empty if non set will work.
Below is your code updated with idea described.
<?php
class home_main_post_below_list extends AQ_Block {
//set and create block
function __construct() {
$block_options = array(
'name' => 'Home main post with below list post',
'size' => 'span12',
);
//create the block
parent::__construct('home_main_post_below_list', $block_options);
}
//create form
function form($instance) {
$titles = isset($instance['titles']) ? esc_attr($instance['titles']) : 'Recent Posts';
$number_show = isset($instance['number_show']) ? absint($instance['number_show']) : 5;
?>
<p><label for="<?php echo $this->get_field_id('titles'); ?>"><?php _e('Title:', 'tl_back'); ?></label>
<input class="widefat" id="<?php echo $this->get_field_id('titles'); ?>" name="<?php echo $this->get_field_name('titles'); ?>" type="text" value="<?php echo $titles; ?>" /></p>
<p><label for="<?php echo $this->get_field_id('number_show'); ?>"><?php _e('Number of posts to show:', 'tl_back'); ?></label>
<input id="<?php echo $this->get_field_id('number_show'); ?>" name="<?php echo $this->get_field_name('number_show'); ?>" type="text" value="<?php echo $number_show; ?>" size="3" /></p>
<label for="<?php echo $this->get_field_id('cats'); ?>"><?php _e('Select categories to include in the recent posts list:', 'tl_back'); ?>
<?php
$categories = get_categories('hide_empty=0');
echo "<br/>";
foreach ($categories as $cat) {
$option = '<input type="checkbox" id="' . $this->get_field_id('cats') . '[]" name="' . $this->get_field_name('cats') . '[]"';
if (isset($instance['cats'])) {
foreach ($instance['cats'] as $cats) {
if ($cats == $cat->term_id) {
$option = $option . ' checked="checked"';
}
}
}
$option .= ' value="' . $cat->term_id . '" />';
$option .= ' ';
$option .= $cat->cat_name;
$option .= '<br />';
echo $option;
}
?>
</label>
<?php
}
//create block
function block($instance) {
extract($instance);
$titles = apply_filters('widget_title', empty($instance['titles']) ? 'Recent Posts' : $instance['titles'], $instance, $this->id_base);
if (!isset($instance["cats"])) {
$cats="";}
// array to call recent posts.
$themeloy_args = array(
'showposts' => $number_show,
'category__in' => $cats,
);
$themeloy_widget = null;
$themeloy_widget = new WP_Query($themeloy_args);
?>
<div class="widget post_list_medium_widget">
<?php if (!empty($instance['titles'])) {?><h3 class="widget-title"><span><?php echo $instance["titles"];?></span></h3><?php }?>
<div class="widget_container">
<div class="post_list_medium">
<?php
$i = 0;
while ($themeloy_widget->have_posts()) {
$i++;
$themeloy_widget->the_post();
$post_id = get_the_ID();
$thumb = themeloy_get_thumbnail(get_the_ID());
if ($i == 1) {
?>
<div class="list_item">
<div class="entry-thumb feature-item">
<a href="https://wordpress.stackexchange.com/questions/360692/<?php the_permalink(); ?>">
<?php if ( has_post_thumbnail()) {
echo wp_get_attachment_image( get_post_thumbnail_id( $post_id ), 'feature-large', false, array('class'=>'cat-feature-large-img') );
} else {echo '<img class="cat-feature-large-img" src="'.get_template_directory_uri().'/images/demo/feature-large.jpg'.'" alt="">';} ?>
</a>
<div class="cat-feature-large">
<?php $post_cat = get_post_custom_values('cat_themeloy_select', get_the_ID());
$post_cat = ($post_cat[0] != '') ? $post_cat[0] : of_get_option('blog_category_post');
if($post_cat == '1'){echo '<p class="cat-slider">'; echo the_category(', ').'</p>';}?>
</div>
<?php echo themeloy_post_type(); ?>
</div>
<h2><a href="https://wordpress.stackexchange.com/questions/360692/<?php the_permalink(); ?>" class="title"><?php the_title(); ?></a></h2>
<?php echo themeloy_post_meta(get_the_ID());?>
<p><?php echo themeloy_short_title(320, get_the_excerpt('')); ?> </p>
</div>
<div class="clear margin-buttons"></div>
<?php }else{?>
<div class="small-list-content">
<?php
$post_date = get_post_custom_values('date_themeloy_select', get_the_ID());
$post_date = ($post_date[0] != '') ? $post_date[0] : of_get_option('blog_date_post');
if($post_date == '1'){
?>
<div class="feature_post_style">
<span class="post_date"><span class="date_number"><?php echo get_the_date('d');?></span> <?php echo get_the_date('M');?>
<i class="icon-caret-right feature-icon-right"></i>
</span>
<span class="post_time"><i class="icon-time"></i> <?php echo get_the_time('H:i');?></span>
</div>
<?php }?>
<div class="feature-link feature-item">
<a href="https://wordpress.stackexchange.com/questions/360692/<?php the_permalink(); ?>" class="feature-link">
<?php if ( has_post_thumbnail()) {
echo wp_get_attachment_image( get_post_thumbnail_id( $post_id ), 'medium-feature', false );
}else{echo '<img src="'.get_template_directory_uri().'/images/demo/medium-feature.jpg" alt="">';} ?>
</a>
<?php echo themeloy_post_type(); ?>
</div>
<div class="list_desc">
<h4 class="list_title"><a class="post-title" href="https://wordpress.stackexchange.com/questions/360692/<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title_attribute(); ?>">
<?php the_title(); ?>
</a></h4>
<?php echo themeloy_post_meta(get_the_ID());?>
</div>
<div class="clear"></div>
</div>
<?php }}?>
</div>
</div>
</div>
<?php
wp_reset_query();
}
function update($new_instance, $old_instance) {
return $new_instance;
}
}
Also fallback img tags if none post_thumbnail set got alt attribute empty as it’s required by accessibility guidelines.