Display images in template file

if You wish to display an image, which is a thumbnail of a post, then use:

  <?php echo get_the_post_thumbnail( $pageID, 'thumbnail' ); ?>

if You wish to display an image from media, then use

<?php echo wp_get_attachment_image( $attachmentID, 'thumbnail' ); ?> 

thumbnail is set for size and is set by default. Size can be changed to medium,large or full

if You want to display a specific image, just insert the img tag

There are more ways, this are the basic ones, which are enough. Good luck