We received the same complaint from another user too and while investigating the issue, we found that Vimeo had an issue temporarily with their OEMBED API that returned no image URLs for their videos. But, the issue seemed to be solved now.
Solution:
1. Edit open the videos that don't have images from your Joomla! back-end (Components => All Video Share => Videos)
2. Locate the "Image" field. Make it empty and save the form again.
3. This should auto-generate a correct image URL for your Vimeo video.
Hope this solved your issue!
In case the issue remains, kindly share with us your site admin access details here
allvideoshare.mrvinoth.com/#contact-us
to further investigate the issue.