You can get the skin from the download path to your RadControls. For example:
C:\Program Files (x86)\Telerik\RadControls for ASP.NET AJAX Q3 2010\Skins\Sitefinity
Here's the Css code to set the new image as right arrow:
.RadScheduler_Sitefinity .rsApt .rsArrowRight
Please find the new arrow in the attached .zip file and let me know if this was helpful.
the Telerik team