Share It - Setting a Custom Title, URL or Image
Open the Page Inspector on the page the Share It stacks is on. Then click on the Header tab on top. You will need to create up to three new meta tags, depending on what you want to make custom.
The three names of the tags will as follows:
Then for the content of the Meta Tags, you just fill in the appropriate information. You will need to use a URL for the image, so it has to be hosted on your server. Remember, this might take a bit for services like Facebook to cache the image, so do not expect to see it right after publishing.
Now go forth and make your websites great!