Basically keeping the link thumbnail saved on some relays like Damus already does. With Primal, all thumbnails from nostr:npub1qfv2tudq6uawsutmzal5x8me5y26692xxfvf587wg79tl6l9q4msp5uyv9 are not shown because everytime Primal calls back to the website server to retrieve the image and is flagged as a bot.