A link preview needs two things: the right tags in your page's <head>, and an image the platform's crawler can download. Mosaic handles the image. This page covers the tags and how to check the result.

The tags to add

Add both the Open Graph and the X (Twitter) tags. X falls back to og:image, but twitter:card is what makes it show a large image instead of a small thumbnail.

<meta
  property="og:image"
  content="https://mosaic.praveenjuge.com/use?url=https%3A%2F%2Fexample.com%2Fpricing"
/>
<meta property="og:image:width" content="1560" />
<meta property="og:image:height" content="819" />
<meta name="twitter:card" content="summary_large_image" />
<meta
  name="twitter:image"
  content="https://mosaic.praveenjuge.com/use?url=https%3A%2F%2Fexample.com%2Fpricing"
/>

On the dashboard, Get code on a website builds these tags for any page: type the page's path and copy them. The url parameter is the page's full address, URL-encoded. Use the page's canonical address, the one people share, so every share reuses one image.

Image size and format

Size: 1560×819 pixels, the 1.91:1 shape Facebook, LinkedIn, X, Slack, and Discord use for large link previews. The width and height tags above let some platforms lay out the preview before the image downloads.
Format: JPEG at quality 80, a few hundred KB at most, well under every platform's limit (X's is 5 MB).
What's in it: the top of the page, as a visitor sees it on a wide screen. Cookie banners, chat widgets, and analytics scripts are left out.

How crawlers get the image

/use answers with a 307 redirect to the image's permanent address under /i/. Every major crawler follows redirects. The first request for a new page takes the screenshot, which can take a few seconds; later requests get the saved image at once.

Check a preview

Each platform caches previews, often for days. After you change a tag, ask the platform to fetch the page again:

Facebook, Instagram, Threads, WhatsApp: Sharing Debugger. Choose Scrape Again to refresh its copy.
LinkedIn: Post Inspector. Inspecting a URL refreshes LinkedIn's cache.
X: there's no public validator. Paste the link in a new post and wait for the preview. X can keep an old preview for about a week.
Slack and Discord: paste the link in a message.
Any platform: metatags.io and opengraph.xyz show how a page will look on several platforms at once.

To check the image itself, open the /use link in a browser. You should see the screenshot.

Common problems

No image at all: make sure the tag is in the HTML the server sends, not added later by JavaScript. Crawlers don't run scripts. View the page source to check.
A grey or example image: the hostname isn't saved in Mosaic, or a daily limit was reached before the first screenshot. Add the website on the dashboard, then see Understanding Usage Limits.
An old screenshot: images refresh after 30 days. To update one now, use Regenerate next to it on the dashboard, then scrape the page again in the platform's debugger. See How Mosaic Refreshes OG Images.
A small square thumbnail on X: add twitter:card with the value summary_large_image.
The wrong page in the image: the url parameter must be the page's own address, encoded. An unencoded ? or & in it cuts the address short.