Instead of using the code like this
var r = window.confirm("Are you sure you want to Continue without Selecting Image?");
if (r == true)
why dont you try using it like this ??
if (confirm("Are you sure you want to Continue without Selecting Image?")==true)
return true;
else
return false;
This code is working file in my browser. So give it a try.
BR//
Harsha