Struts 2 tags
are evaluated at page render time, which is far before your script runs.
If you check the source code of the page you will see those tags have been converted to regular html elements. So it the para
javascript variable would not be evaluated.
You can try to update the return line to
return "<a style='cursor:pointer;' onClick='javascript:openDialog("+cellvalue+")'><img src='<s:url action='ImageAction' namespace='/myimage/secure'></s:url>" + "?imageId="+para+ "'></a>";