thumbnail path problem with asp.net ~ symbol

Hi, all.
i configed BaseUrl = "~/proimg/"; in ckfinder\config.ascx
image upload and select works fine.
when i use "Select" the url is right like /product/proimg/image/face.jpg,
but the thumbnail is a question mark and "Select thumbnail" returns a wrong url like ~/proimg/_thumbs/Images/face.jpg
obviously the ~ symbol resolved in the server side and transfered to client side.
i think it just a little mistake in the js file.
can you fix it?