Ensure that both ImageMagick and Ghostscript are in the systems PATH variable (My Computer > Properties > Advanced > Environment Variables. Path is in the bottom under System Variables).

This works from the command line but not through the application - see this article for further detail 

If it does not work from the command line or the application:
- Please make sure that you install Ghostscript prior to installing ImageMagick.

We have seen cases where thumbnails for PDF files are not created because of this: Postscript delegate failed : No such file or directory error.

In most cases uninstalling ImageMagick and reinstalling it will solve the problem.