Yes. Open the video from the gallery, and the Edit Media panel has a Video thumbnail section under the title and description. Choose Upload thumbnail, pick an image, and it replaces the frame UNI picked for you. Once one is set the button reads Replace thumbnail, so you can change it as often as you like.
The rules are narrow, so check them before you export:
- JPEG or PNG only. Anything else gives Thumbnails must be JPEG or PNG.
- Under 10 MB. Anything larger gives Thumbnails must be under 10 MB.
- There is no minimum size, but the thumbnail is displayed at the video's own proportions, so match the video's shape or it will be cropped.
A successful upload says Thumbnail updated and the preview beside the button changes at once.
The automatic thumbnail arrives with processing, not with the upload
When you upload a video, UNI takes a still from it and uses that. That still is only produced once processing finishes, which is why a freshly uploaded video shows No thumbnail yet for a while. If you upload your own before processing has finished, yours is kept: an uploaded thumbnail overrides the automatic one rather than the other way round.
The automatic still is reduced to 1280 pixels on its longest edge, so a thumbnail you upload yourself does not need to be any larger than that to look sharp.
The change is live straight away
A thumbnail is stored on the video itself rather than in your draft, so it does not wait for Publish. Visitors see the new thumbnail as soon as it uploads. That is worth knowing before you experiment on a project that is already public.
Only the person who uploaded the video can change its thumbnail
Thumbnails are tied to whoever uploaded the video, not to the project team. A collaborator who can otherwise edit the project will be told the video cannot be found when they try. If the wrong thumbnail is on a teammate's video, ask them to replace it, or delete the video and upload it again yourself.
A video cannot be your project cover
You will notice there is no Set as cover image option on a video, and this is deliberate rather than an oversight. The cover is always a still image, and a project whose only media is video shows no cover at all. If you want a frame of the video representing the project, export that frame and upload it as an image, then tick Set as cover image on it. Deleting the cover image explains how the cover is chosen.