When you share a link on facebook it will automatically find images on the website and randomly picks one as a preview. How can you influence the preview image? When a person shares the website link on his facebook?
1. Include the Open Graph XML namespace extension to your HTML declaration
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:fb="http://ogp.me/ns/fb#">
2. Inside your <head></head> use the following meta tag to define the image you want to use
<meta property="og:image" content="fully_qualified_image_url_here" />
Read more about open graph protocol here.
After doing the above, use the Facebook "Object Debugger" if the image does not show up correctly. Also note the first time shared it still won't show up unless height and width are also specified, see Share on Facebook - Thumbnail not showing for the first time
Note also that if you have wordpress just scroll down to the bottom of the webpage when in edit mode, and select "featured image" (bottom right side of screen).
If you're using Weebly, start by viewing the published site and right-clicking the image to Copy Image Address. Then in Weebly, go to Edit Site, Pages, click the page you wish to use, SEO Settings, under Header Code enter the code from Shef's answer:
<meta property="og:image" content="/uploads/..." />
just replacing /uploads/... with the copied image address. Click Publish to apply the change.
You can skip the part of Shef's answer about namespace, because that's already set by default in Weebly.
来源：https://stackoverflow.com/questions/6693499/how-can-i-set-a-website-image-that-will-show-as-preview-on-facebook