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