When making a database of your videos on your hard drive, you should consider adding thumbnails of that video in the database so that next time you are going through your video list, you can quickly find the one you are looking for just by looking at the video thumbnails.
Even though you can use Media Player Classic (MPC-HC) for creating the thumbnails of videos and save it to a PNG file, it does not give you very much control over the whole process. If you want more options and customized thumbnail grid creation for any video, then you can make use of another free software called MoviePrint. It lets you create gallery of screenshots taken at regular intervals of an entire video.
The software is designed using Electron framework and is available for both macOS and Windows. After launching it, you can add one or more than one videos to the list. Once you have added the videos, you can change how the thumbnails are generated from the right side of the MoviePrint interface. You can change the number of columns, number of rows and number of thumbs. You can add a white paper background and choose a layout of the thumbnail gallery.
It will generate the thumbnails as soon as you change the settings or add the videos. For each of the thumbnail, you can select a custom frame from the video, you can save the individual thumbnail as PNG image, you can shift the thumbnail to a different position, you can insert new thumbnails before or after etc.
You can export the thumbnail gallery (or MoviePrints) as PNG files. When you click on the save MoviePrint button (or use the hotkey M) it will save the thumbnail files on the Windows desktop.
You can download MoviePrint from https://movieprint.fakob.com/.