Post Thumbnails

Post Thumbnails

Sample Functions.php:

if ( function_exists( 'add_theme_support' ) ) { 
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150, true ); // default Post Thumbnail dimensions (cropped)

// additional image sizes
// delete the next line if you do not need additional image sizes
add_image_size( 'category-thumb', 300, 9999 ); //300 pixels wide (and unlimited height)
}


in File :


 <?php the_post_thumbnail( $size, $attr ); ?> 





Read More:
http://codex.wordpress.org/Function_Reference/the_post_thumbnail
https://codex.wordpress.org/Post_Thumbnails