You make the thumbnail in 1920x1080 or 1280x720 or 1x1 ratio depending on your video dimensions and add picture in video editing software and add video after it. So basically merge image and video so that image is taking 1 second of the video at start. To further be clear check it on my gig, watch the video and if you pause it at start you will see the thumbnail image https://www.fiverr.com/gamezerker/edit-your-fortnite-gameplay?context_referrer=search_gigs&context_type=auto&pckg_id=1&pos=1&ref_ctx_id=34db9bdc-bdb1-4b7a-9cf6-244dbde94725&seller_online=true&funnel=18a32e8c-c50f-43ef-b087-40a99692964d once they approve your video on the gig you can go into the gig itself and click pencil icon on the video to change the thumbnail by selecting a frame in your video, and you just go and pause the video at start and select that as thumbnail. Let me know if this helps.