Tooltip isn't showing when position is set

Question

I'm using jQuery UI to show tooltip with image and description. I want change position of tooltip. But when I use "position" option tooltip doesn't show. Code is below.

Without positionig:

$( "#vip" )
  .tooltip({ content: '<img src="img/sale/taneczna.jpg" /> Description of image'})

with:

$( "#vip" )
  .tooltip({ content: '<img src="img/sale/taneczna.jpg" /> ( mieści ok. 30 osób)', 
  position: { my: "top right", at: "right center" }})

How to change tooltip position?