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Set {position: 1} to make the tooltip stays in the center of the screen, as illustrated on page http://www.menucool.com/tooltip/javascript-tooltip#view2 (The section of "Tooltip Options" and "Override Global Options").
For example, if the center-position is only for a specific image and you don't want to change the global tooltip options, you can:
<img src="..." onmouseover="tooltip.pop(this, '#tip3', {position:4})" />