That's because you left out the href="" attribute, so the links points to the top of the current page.

Add the code in red to your link and the browser won't actually follow the link.

<a href="" onClick="window.open('newpage.html','mywindow','wi dth=400,height=200'); return false;" onmouseover="MM_swapImage('Image1','','mypic.gif', 1)" onmouseout="MM_swapImgRestore()"><img src="mypic2.gif" name="Image1" width="56" height="27" border="0" id="Image1" /></a>