Check the picture's file format is JPEG or PNG and that the image URL is still live.
Ensure that your account is authorized for posting in HubSpot. To do this, connect the account again.
Third party network issue
Check that your image is valid and your account is authorized as per the above instructions. Once done, clone your social post and publish it again. If you encounter the same issue, you may want to get help with HubSpot.
Facebook rate limit
Facebook has a limit on their API. This can affect posting to your Facebook account through HubSpot. The best solution is to wait, as this limit adjusts over a 24-hour period.