Hi Sarah,
We were able to circumvent the issue by building our images in the following dimensions:
Facebook/LinkedIn: 1200x628
Twitter: 800x418
(I use bannersnack.com to build/export the images and save as templates. Tested the dimensions above on our social posts through HubSpot this morning; the previews were still inconsistent --i.e., showed grey bars for FB and Twitter-- but the actual posts looked correct.)
Hopefully HubSpot will fix the preview bug, but in the meantime this workaround should help! 
Best,
Andrea