I have searched around the internet, but even YouTube's own website lists different resolutions and aspect ratios for YouTube Video Thumbnails. So, from personal experience perhaps, can anyone help me?

The most important part of the Thumbnail is the snippet of the video that you are showing. Think of it like a 1 second preview of the video. People who do series of videos sometimes use a Title Screen here. Once you pick the scene, then pick the resolution that best shows what is in the video (more detail needs increased resolution). The Aspect Ratio should be the same as the original video

I'm not picking a scene from the actual video, but instead creating generic thumbnails which can be reused on different video series which a friend of mine runs on his YouTube channel. For example, he has a Q&A series, so I'll do one for that. The videos are each filmed with different cameras, some are just webcam vlogs or others filmed with professional equipment, so I can't simply resolution match them either.