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Okay, I have answers. 

The minimum image size for the Facebook to share any image is 200x200, but that is just the minimum for any picture to show up - not for the ideal size. The minimum ideal size is no less than 600x314, and optimal size is 1200x630 - in other words, the ideal image for Facbeook share is formatted for a rectangle. If we want the share module to use the whole image without cropping, we need to make sure each image is as close to 600x314 as possible.

Per Facebook, "At the minimum, you should use images that are 600 x 315 pixels to display link page posts with larger images." and " Try to keep your images as close to 1.91:1 aspect ratio as possible to display the full image in News Feed without any cropping."

The problem cannot be fixed by changing the Miva social share tool. It has to be fixed by actually loading a picture that is optimized for Facebook's share dimensions. 

Here is the documentation from Facebook: https://developers.facebook.com/docs/sharing/best-practices