jQuery Grid Plugin - jqGrid

Up to jqGrid 3.4, here was the way to add a custom button in the navigator bar of a jqgrid:

jQuery("#grid_id").navGrid("#pager",{standard parameters}).navButtonAdd("#pager",{custom parameters});
The Custom parameters are

{ 
   caption:'NewButton', 
   buttonimg:'full_path_to_image.gif', 
   onClickButton:null, 
   position "last", 
   title:'ToolTip' 
}

Since jqGrid 3.5, the parameter 'buttonimg' has no effect anymore.

Is there a good reason for this new behavior ?

Thanks for your help.