Your approach appears to be correct and it works on my end as you can check in the attached sample page. Most probably the problem at hand stems from the data that is loaded in the RadRotator control.
You mentioned that the same picture is loaded in all items of the rotator. Is it a picture from the data source or is it photoempty_v.jpg? In case the photoempty_v.jpg file is loaded, most probably the path you pass to the MapPath method is not a valid one.
If the image that is loaded in the rotator is from the data source, please check the data source and verify that it indeed contains different image file names and not the same one in all records.
If you are still having difficulties after checking the information above, try reproducing the problem you encountered in the attached sample page and send it so that I can inspect it locally and provide a more to the point answer.
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.