When you upload a video, you can choose which frame from the video is used as its thumbnail, in Feed and Chat videos. You can also come back and change it later.
This feature is available to everyone, on all platforms, with no setup required.
Choose a thumbnail when you upload
As soon as you upload a video, you'll be prompted to pick a thumbnail from a selection of frames taken from the video. Choose the one you'd like to use, and continue with your post as normal.
Change the thumbnail later
If you want to update the thumbnail after the video has been posted:
Find the post and click the three dots (⋯) on the video.
Select Choose thumbnail.
Pick a new frame from the video.
You can repeat these steps as many times as you like until you're happy with the result before publishing. Once it's published you can't change the thumbnail.