A thumbnail /cover image can be changed but I do not know how this behaves in various ways that you may wish to use it.
So bear in mind this may not give you what you are wanting from this post.
Open your .mov file in QuickTime .Go to ( pause on) the frame that you want, in your case you say you want the first image.
Now select Edit >Copy.
Now click on your .mov file and then select File >Get info.
Click on the image in the top left and a blue border will be placed around it.
Now select Edit > Paste
Your cover image should now change.
You can even copy/paste from a different .mov file, or even copy/paste from a .jpg .
As I say, I am not sure how this behaves when you upload this modification to your Google drive.
