Source

File: wp-includes/class-wp-theme.php

public function get_screenshot( $uri = 'uri' ) {
		$screenshot = $this->cache_get( 'screenshot' );
		if ( $screenshot ) {
			if ( 'relative' === $uri ) {
				return $screenshot;
			}
			return $this->get_stylesheet_directory_uri() . '/' . $screenshot;
		} elseif ( 0 === $screenshot ) {
			return false;
		}

		foreach ( array( 'png', 'gif', 'jpg', 'jpeg' ) as $ext ) {
			if ( file_exists( $this->get_stylesheet_directory() . "/screenshot.$ext" ) ) {
				$this->cache_add( 'screenshot', 'screenshot.' . $ext );
				if ( 'relative' === $uri ) {
					return 'screenshot.' . $ext;
				}
				return $this->get_stylesheet_directory_uri() . '/' . 'screenshot.' . $ext;
			}
		}

		$this->cache_add( 'screenshot', 0 );
		return false;
	}