Show us your code and be more specific about exactly what you want to have happen. Also, include a link to your current page. In any case, to open a link in a new window you can use the the window.open() method, ex:
Code:
<a href="big_picture.jpg" onclick="window.open(this.href,'','width=300, height=250');return false;" target="_blank"><img src="small_picture.jpg" border="0" onmouseover="your mouseover code here" onmouseout="your mouseout code here"></a>
But this raises alignment issues especially in IE in the new window. So, more elaborate code can be used, usually best written in a script elsewhere on the page as a function and called with parameters onclick. Or you can put the large image on a seperate page and use that page for the href. Then the alignment can be dealt with in the HTML code of that separate page.
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