WordPress/wp-admin/js/custom-background.js

(function($) {
	$(document).ready(function() {
		var bgImage = $("#custom-background-image"),
			frame;

		$('#background-color').wpColorPicker({
			change: function( event, ui ) {
				bgImage.css('background-color', ui.color.toString());
			},
			clear: function() {
				bgImage.css('background-color', '');
			}
		});

		$('input[name="background-position-x"]').change(function() {
			bgImage.css('background-position', $(this).val() + ' top');
		});

		$('input[name="background-repeat"]').change(function() {
			bgImage.css('background-repeat', $(this).val());
		});

		$('#choose-from-library-link').click( function( event ) {
			var $el = $(this);

			event.preventDefault();

			// If the media frame already exists, reopen it.
			if ( frame ) {
				frame.open();
				return;
			}

			// Create the media frame.
			frame = wp.media({
				// Set the title of the modal.
				title: $el.data('choose'),

				// Tell the modal to show only images.
				library: {
					type: 'image'
				},

				// Customize the submit button.
				button: {
					// Set the text of the button.
					text: $el.data('update'),
					// Tell the button not to close the modal, since we're
					// going to refresh the page when the image is selected.
					close: false
				}
			});

			// When an image is selected, run a callback.
			frame.on( 'select', function() {
				// Grab the selected attachment.
				var attachment = frame.state().get('selection').first();

				// Run an AJAX request to set the background image.
				$.post( ajaxurl, {
					action: 'set-background-image',
					attachment_id: attachment.id,
					size: 'full'
				}).done( function() {
					// When the request completes, reload the window.
					window.location.reload();
				});
			});

			// Finally, open the modal.
			frame.open();
		});
	});
})(jQuery);