It sounds like the logo for your website is a rectangle, while Facebook is expecting a square image for the share. To resolve this, there are two options: 

1) You can edit the social share image of all of your pages under the Settings > Social section of each page. A "thumbnail" image can be uploaded specific to each page. If these share images do not post correctly immediately, you can use Facebook's debugger tool to kickstart the process. 

2) You can modify your custom theme to hard-code your desired logo onto your website, and update the logo set in your control panel (Website > Site settings > Logo) to the image you'd like to be used for social media. 

In a bootstrap-based theme, for example, you'll find where the logo is pulled in on line 20 of the _nav.html file. The code you'll need to replace with your image URL is {{ site.meta_image_url }}. To obtain a URL for your image, you can upload the image to your theme files and then use the name of the image (i.e. src="myLogo.png").

If you have specific questions about making this modification in your theme file, please feel free to reach out to our support team at [email protected].