the_post_thumbnail()

the_post_thumbnail() — Displays the post thumbnail. When a theme adds ‘post-thumbnail’ support, a special ‘post-thumbnail’ image size is registered, which differs from the ‘thumbnail’ image size managed via the Settings > Media screen. When using the_post_thumbnail() or related functions, the ‘post-thumbnail’ image size is used by default, though a different size can be specified instead as needed.

Inputs: $size (string|int[]): Optional. Image size. Accepts any registered image size name, or an array of width and height values in pixels (in that order). Default ‘post-thumbnail’. $attr (string|array): Optional. Query string or array of attributes. Default empty.