I rate 2JPEG and it seems to support 275 formats , including Word, Excel, Publisher and Powerpoint files. fCoder recommends running 2JPEG as a scheduled background task. command line syntax is pretty detailed.
:
2jpeg.exe -src "c:\files\myfile.docx" -dst "c:\files" -oper Resize size:"100 200" fmode:fit_width -options pages:"1" scansf:no overwrite:yes template:"{Title}_thumb.jpg" silent:yes