jQuery(document).ready(function() { 

  jQuery('.sponsor img').each(function(){
      var t=jQuery(this);
      
      var src1= t.attr('src'); // initial src
      var filename = src1.substring(0, src1.lastIndexOf('.')); // cuts of everything after last dot
      var extension = /[^.]+$/.exec(src1);
            
      t.hover(
        function(){
          t.attr('src', filename + '-over.' + extension);
        },
        
        function(){
          t.attr('src', filename + '.' + extension);
        }
      );
  });

});