JAMIN

While developing wordpress theme, you wont get section to add post image. So you need to add the following codes to function.php

To display thumbnail in single.php or page.php (anywhere in frontend), you need to insert this code inside php

// Add feaure image thumbnail
function thumbnail_add(){
add_theme_support( 'post-thumbnails' );
}
add_action("after_setup_theme","thumbnail_add");

add_action is an action hook we use “after_setup_theme” as action hook, and thumbnai_add is function name above

You can use the following code in function.php for different pages

the_post_thumbnail();

1. EVERYWHERE

add_theme_support( 'post-thumbnails' );

2. ONLY PAGES

add_theme_support( 'post-thumbnails', array( 'page' ) );

3. ONLY POSTS

add_theme_support( 'post-thumbnails', array( 'post' ) );

O3. NLY CUSTOM POST TYPES

add_theme_support( 'post-thumbnails', array( 'your-post-type-name' ) );

The example of array are as follow which might be the possible arrays

array(
'height'=>50,
'width'=>200,
'default-image' => '',
'default-preset' => 'default', // 'default', 'fill', 'fit', 'repeat', 'custom'
'default-position-x' => 'left', // 'left', 'center', 'right'
'default-position-y' => 'top', // 'top', 'center', 'bottom'
'default-size' => 'auto', // 'auto', 'contain', 'cover'
'default-repeat' => 'repeat', // 'repeat-x', 'repeat-y', 'repeat', 'no-repeat'
'default-attachment' => 'scroll', // 'scroll', 'fixed'
'default-color' => '',
'wp-head-callback' => '_custom_background_cb',
'admin-head-callback' => '',
'admin-preview-callback' => '',
);

CUSTOM LOGO
You can also customize your thumbails that is small size or banner size. you can use the following code in function.php

add_image_size('small-thumbnail',12,90,true);

where 120 is width , 90 is height, true gives cropping value true whereas false denote no cropping. ‘small-tumbnail’ is name of the thumbnail you can use while calling.

in your page (index.php,single.php,page.php…) you can call the custom thumbnail by using this code

the_post_thumbnail('small-thumbnail');