So I used the codes below in the functions.php
add_theme_support( 'post-thumbnails', array( 'post')); set_post_thumbnail_size(100, 75);
And then in THE LOOP I called:
<?php the_post_thumbnail(); ?>
The image has an entry in the database with its url but the file isn't uploaded when I checked in the wp-content/uploads/ directory.
What am I doing wrong?