Icaros is a collection of lightweight, high-quality, Windows Shell Extensions, which is capable of providing Windows Explorer thumbnails for essentially any video media file type. Icaros can provide Windows Explorer thumbnails, for essentially any video media format supported by FFmpeg which includes popular file types such as: MKV, FLV, AVI, MP4, MOV, RMVB, M2TS, OGM, etc. It is capable of thumbnailing just about any file by adding its extension(s) to the optional filetypes box and re-activating Icaros.
To set an offset to use for video thumbnails, after installing, open Icaros → Thumbnailing → Activate → SETTINGS → Set the "Thumbnail Offset." By default, it is set to 25% progress of the video. You can change this percentage, or you can also opt to set a specific preferred timestamp.