What is currently the best website thumbnail generator?

Working on a new project and would like to include website thumbnails, preferably in an automated process in which thumbnails can be periodically updated. What is the best generator? I have looked at half a dozen online services and they either dont work (many dont seem to be maintained) or work only partially.

If there are no current online services for the job, I guess im looking at buying a script of some sort.... WordPress ...