FileNotFound exception when trying to overwrite or delete an image in CF8. Why?

I have CF8 Standard Edition installed in my server and when I try to use CFIMAGE to overwrite the uploaded file with a resized one, or to write an image previously resized with ImageResize or ImageScaleToFit, I get an internal server error caused by a java.io.FileNotFoundException with some images, many in fact. In other words, the error happens because the system cannot find the file to overwrite.

Apparently the process jrun.exe maintains the original image blocked in such a way that even rejects a file name change. I used the same CF version in an older server and everything run smoothly, but when I had to change the server and migrate my websites I found this problem. The server is IIS7 on a Windows Server 2008 OS. Of couse any CMS without a proper image management is unuseful, so that is a critical issue for me.

Can somebody help me?