Given a video file, ffthumb chooses a thumbnail image by extracting the first few keyframes and selecting the largest and least compressible file.

ffmpeg is used for keyframe extraction and is not provided.

ffprobe is used to determine if aspect ratio correction is required, and is also not provided.

Documentation: https://godoc.org/github.com/7fffffff/ffthumb

Sometimes the first frame makes for a poor thumbnail, because it's all one color or something like that.