Source

File: wp-includes/widgets/class-wp-widget-media-image.php

public function get_instance_schema() {
		return array_merge(
			array(
				'size'              => array(
					'type'        => 'string',
					'enum'        => array_merge( get_intermediate_image_sizes(), array( 'full', 'custom' ) ),
					'default'     => 'medium',
					'description' => __( 'Size' ),
				),
				'width'             => array( // Via 'customWidth', only when size=custom; otherwise via 'width'.
					'type'        => 'integer',
					'minimum'     => 0,
					'default'     => 0,
					'description' => __( 'Width' ),
				),
				'height'            => array( // Via 'customHeight', only when size=custom; otherwise via 'height'.
					'type'        => 'integer',
					'minimum'     => 0,
					'default'     => 0,
					'description' => __( 'Height' ),
				),

				'caption'           => array(
					'type'                  => 'string',
					'default'               => '',
					'sanitize_callback'     => 'wp_kses_post',
					'description'           => __( 'Caption' ),
					'should_preview_update' => false,
				),
				'alt'               => array(
					'type'              => 'string',
					'default'           => '',
					'sanitize_callback' => 'sanitize_text_field',
					'description'       => __( 'Alternative Text' ),
				),
				'link_type'         => array(
					'type'                  => 'string',
					'enum'                  => array( 'none', 'file', 'post', 'custom' ),
					'default'               => 'custom',
					'media_prop'            => 'link',
					'description'           => __( 'Link To' ),
					'should_preview_update' => true,
				),
				'link_url'          => array(
					'type'                  => 'string',
					'default'               => '',
					'format'                => 'uri',
					'media_prop'            => 'linkUrl',
					'description'           => __( 'URL' ),
					'should_preview_update' => true,
				),
				'image_classes'     => array(
					'type'                  => 'string',
					'default'               => '',
					'sanitize_callback'     => array( $this, 'sanitize_token_list' ),
					'media_prop'            => 'extraClasses',
					'description'           => __( 'Image CSS Class' ),
					'should_preview_update' => false,
				),
				'link_classes'      => array(
					'type'                  => 'string',
					'default'               => '',
					'sanitize_callback'     => array( $this, 'sanitize_token_list' ),
					'media_prop'            => 'linkClassName',
					'should_preview_update' => false,
					'description'           => __( 'Link CSS Class' ),
				),
				'link_rel'          => array(
					'type'                  => 'string',
					'default'               => '',
					'sanitize_callback'     => array( $this, 'sanitize_token_list' ),
					'media_prop'            => 'linkRel',
					'description'           => __( 'Link Rel' ),
					'should_preview_update' => false,
				),
				'link_target_blank' => array(
					'type'                  => 'boolean',
					'default'               => false,
					'media_prop'            => 'linkTargetBlank',
					'description'           => __( 'Open link in a new tab' ),
					'should_preview_update' => false,
				),
				'image_title'       => array(
					'type'                  => 'string',
					'default'               => '',
					'sanitize_callback'     => 'sanitize_text_field',
					'media_prop'            => 'title',
					'description'           => __( 'Image Title Attribute' ),
					'should_preview_update' => false,
				),

				/*
				 * There are two additional properties exposed by the PostImage modal
				 * that don't seem to be relevant, as they may only be derived read-only
				 * values:
				 * - originalUrl
				 * - aspectRatio
				 * - height (redundant when size is not custom)
				 * - width (redundant when size is not custom)
				 */
			),
			parent::get_instance_schema()
		);
	}