Dallas here from the Social Care Team. That is a very good question and I completely understand how frustrating that is for you! Let me help to explain how it works a bit and then I will help you to fix it all up.
When you make a post to your Facebook page the image, store name, URL, etc. are pulled over to your post. This information is called Open Graph Objects. It is what is pulled through for all social media platforms, not just Facebook. This link here will explain it in greater detail, but I completely understand if that page just looks super overwhelming. It is by no means mandatory to read! I do, however, have this other really helpful link that I also recommend reading through. It also explains the whole process, but it does so in a way that is way easier to digest.
When it comes to actually fixing the issue, if you follow these steps you should be good:
<meta property="og:image" content="X" />
Just replace X with the URL of the image you just uploaded to the File section of your Store Admin.
This should solve the issue for you. If it doesn't or if you have any other questions please don't hesitate to let me know.