Thumbnail Sizes
The default image sizes of WordPress are „thumbnail“ (and its „post-thumbnail“ and „thumb“ aliases), „medium“, „large“ and „full“ (the image you uploaded). These image sizes can be configured in the WordPress Administration Media panel under Settings > Media. This is how you use these default sizes with the_post_thumbnail():
the_post_thumbnail(); // without parameter -> Thumbnail
the_post_thumbnail(‚thumbnail‘); // Thumbnail (default 150px x 150px max)
the_post_thumbnail(‚medium‘); // Medium resolution (default 300px x 300px max)
the_post_thumbnail(‚large‘); // Large resolution (default 640px x 640px max)
the_post_thumbnail(‚full‘); // Original image resolution (unmodified)
the_post_thumbnail( array(100,100) ); // Other resolutions