When you post updates to your LinkedIn Page, you can customize the thumbnail image and title when sharing an article or website URL.

To customize the title and image when posting a URL 

  1. Sign in to your Page admin center.
  2. Paste the URL into the Share an article, photo, video, document, or update box at the top of the page.
    • An auto-generated thumbnail image may appear in the preview if one is available, along with the article or website title.
  3. To customize the image, click the  Image icon below text box and select an image from your computer. To customize the title, click on the current title and type your new title over it.
  4. If you’d like to target your update to a specific audience, click the dropdown menu next in the share box and select Targeted audience. Learn more about targeted updates.
  5. If you’d like the update to display to all Page followers, click the dropdown menu in the share box and select All followers.
  6. Click the Post button to share your update.

When adding an image to your update, please keep the following guidelines in mind regarding image display size:

  • Use a 1.91:1 ratio (1200×627 px).
  • Image must be more than 200px width.
    • If your image width is less than 200px, it will not display in the larger image format. Instead, images will appear as a thumbnail on the left side of the post.
  • Images on mobile will not be cropped. Images of other ratios will show in-full with subtle white padding.

Content Copied from https://www.linkedin.com/help/linkedin/answer/81310/customizing-an-image-and-title-when-posting-a-url-on-your-linkedin-page?lang=en