Not being able to upload a custom thumbnail is one of the most frustrating parts of working with Youtube. A good thumbnail can literally make or break your video.
Reel SEO points out a nice Youtube hack to make your own custom thumbnail in their latest creator’s tip. Tim Schmoyer’s weekly Creator Tips are the best series I’ve come across for Youtube tips and tricks, I definitely recommend checking out his other videos.
Basically it involves adding the image you want for an extended period of time after your actual video. Then upload the video to Youtube and choose your desired thumbnail and take that video in to the Youtube editor and trimm down the length while still keeping your thumbnail.