Thumbnail images

Inside your post loop you can do;

 if ( has_post_thumbnail()) {
 echo get_the_post_thumbnail($post->ID, 'thumbnail');
 }

This first checks for the existence of a featured image and if so it will display one at your default ‘thumbnail’ size as set in your settings/media options.

This would also work within your loop;

 if ( has_post_thumbnail()) {
 the_post_thumbnail('thumbnail');
 }

This would work (but can be used outside of your loop);

 $id = get_the_ID();
 echo get_the_post_thumbnail($id, 'thumbnail');

Beyond that there are other functions to help you customize sizes should that not achieve what you want. However the above defaults to 150×150.

• Read: get_the_post_thumbnail
• Read: the_post_thumbnail

Let me know how you go.