Facebook can be a little (or a lot) annoying with images.
The first time a link is shared and an image selected that will forever and always be the image associated with that FB post.
When you're sharing a post on Facebook, it should show you a box with arrows to select the photo that you want to use - the social media sharing tools do not always show this and will just grab some random image.
To make sure that the correct image is associated with a post. The best way is to login to FB and copy paste the link of the post (you'll want to do this before anyone else has the chance...) - FB will let you choose the image you want to display. If you do not see the correct image, you may need to manually upload the image you want to use to FB yourself.
If you do not see the image that you want to use, you can click on the button at the top of the post edit window to add your own photo.
UPDATE: You can also use this tool to force Facebook to recrawl the page for updated information. You may have to click the "Fetch New Scrap Information" button a few times to get everything but this tool can be a lifesaver!