how to change the thumbnail of my video

[Slowmotiongli]

hello,
i am using after effects 2300 build 59
I would like to change the thumbnail  (poster image) of my video
I tried this: I edit my video in after effect.
I advance the timeline to the image I want to put in thumbnail and my click on stop. a blue vertical line appears.
I click on "time scale" (click to set thumbnail)
I click and nothing happens. I then click on "render" and the video arrives on my desktop, but the thumbnail that appears is not the image I chose on the timeline
could you tell me how to do it?
thank you in advance
cordially

[Mylenium]

There is no such thing. This used to exist for Quicktime files only. For anything else the desktop thumbnail (or for that matter preview on video services) is picked from the file based on rules. Sometimes they'll display the first frame, sometimes the middle frame, sometimes a completely arbitrary frame. Of course services liek YouTube allow to upload a custom preview image. Otherwise you have no control over these things. None whatsoever.

Mylenium

[ShiveringCactus]

Mylenium's right I'm afraid, Poster frames or Thumbanils aren't set in the video, they're set by the container or application that opens the video. It is possible to change the thumbnail on Windows with 3rd party hacks.  This Microsoft post has a video:

https://answers.microsoft.com/en-us/windows/forum/all/how-do-i-change-thumbnail-on-mp4-files-on-my-p... 

On a Mac, it's possible apparently, this is quote from another site:

On Mac, open the video in quicktime and scrub to the frame you want to be your thumbnail. Pause the video and hit command C. This will copy that thumbnail frame. After that, exit out of the quicktime video, right click it and click "Get Info." On the top left corner of the get info page, you will see a small quicktime, vlc, or blank document logo/picture. Click that, then hit command V to paste the thumbnail you've pulled. That should do the trick.