This may be due to something within the data set or the workbook itself as I was unable to reproduce the issue you are experiencing. I attempted that action in the most recent build of Tableau 10 (10000.16.0812.0001) with a somewhat complex workbook and it performed the action successfully and fairly quickly. I was able to successfully past the image into Word.
Did this work in previous version of Tableau with this same workbook? If this is a beta version of Tableau 10, try uninstalling the old version and installing the latest release.
This may be a good case for Tableau Technical Support. They'll likely need a copy of your Desktop Logs to investigate further: Sending Tableau Desktop Log Files | Tableau Software and likely a copy of the workbook as well.
Thanks and hope this helps!