When you change the logo for your site, the old images will still be cached in Facebook. This means that when the URL is shared, the old images will show up. To fix this you will need to re-scrape the images on Facebook.
First you will first need to navigate to this link and log in using Facebook.
Once you are logged in, you will see a screen similar to the one below.
Enter the URL of your site into the textbox.
This will give you the following information, when the images was last scraped, and the image that is currently being used. To update this image, click the “Scrape again” button.