Cool! I had no idea MPC had that. But while a lot of controls are available, the "save thumbnails" actions doesn't seem to be one of them:

.....

Does MPC Home Cinema have more controls?

If you're interested in updating your script so it no longer has to have focus for keypresses, MPC-HC has been updated so that you can now Save Thumbnails from the Web Interface, (VIRTKEY 808).

Media Player Classic - Home Cinema 1.2.908.0

- Add VIRTKEY for "Save Thumbnails" (request #1964399)

For example, the following added to the web interface triggers the Save Thumbnails function:

<noscript>
  <form action="/command.html" method="POST">
    <input type="hidden" name="wm_command" value="808">
</noscript>
    <input type="submit" value="Save Thumbnails" name="submit" onclick="postform(808,'null',0); return false;">
<noscript></form></noscript>

Or, you can now assign a hotkey to it so you don't have to navigate the menu.

Here is a new version of program to control MPC/MPCHC, called "MPCHC kontroler v1.8"
where corrected filenames and other things.

But, much better results (in sense of Robustnes of a used Player, to play all kind of videos)
I achieved with TheKMP, and wrote a control program for it too!
You can read more about it on KMP forum: showthread.php?p=90138#post90138 , or click on link below
http://www.kmplayer.com/forums/showthread.php?p=90138#post90138
Utility program is attached, also in this message, called "KMP Batch Thumbs Automatization v1.7.zip"