Probably been asked before but a quick search didn't turn anything up. Not been using Avidemux long, but I notice the image in some thumbnails are taken from the first frame, while in others it's taken from a frame around 4.5 seconds in. Do I have any control over which frame the thumbnail is taken from?
Video thumbnails are generally out of control of Avidemux as the latter doesn't support adding attachments (https://stackoverflow.com/questions/54717175/how-do-i-add-a-custom-thumbnail-to-a-mp4-file-using-ffmpeg) to video containers which offer such a feature (MP4 and MKV). Still, it is up to the thumbnailer invoked by the file manaher whether to use such an attachment as thumbnail or to generate a thumbnail out of the video stream (I personally would prefer the latter).