jQuery Grid Plugin - jqGrid

If I add to the Bottom navigation it's not change buttonimg
.navButtonAdd('#pager', { caption: "Add", buttonimg: "ui-icon-plus",
     onClickButton: function () {
     PopupCenter('<%= Url.Action("CreatePublisher","Publisher") %>', 'CreatePublisher', '500', '300');
    }, position: "last"
})
But when I add to the Top navigation It changes to ui-icon-plus but when I change caption it's not fix with button

grid.jqGrid('navButtonAdd', '#' + grid[0].id + '_toppager_left', { // "#list_toppager_left"
    caption: "",
    title: "Add Publisher",
    buttonicon: 'ui-icon-plus',
    onClickButton: function () {
    PopupCenter('<%= Url.Action("CreatePublisher","Publisher") %>', 'CreatePublisher', '500', '300');}
});

What I am Wrong ! And Can I add another Image like {add.png,del.png},some Images are not in ui-icon,a custom image.
Thanks in Advance !