The Calendar control uses a DatePicker control internally to select the currently displayed month. You have to follow the instructions from the article that Todor had pointed. You just need to create an Images
folder in your project and put the ok/cancel images there.
Please, let us know if you have further problems displaying the icons.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.