Hi,

In Teams, Adaptive Card images must be fetchable anonymously over HTTPS by the Teams client, using a direct file URL (no cookies, no auth flow, no redirects). Links that work in your browser (because you’re signed in) often fail in Teams. Host the images at a publicly reachable endpoint (e.g., Azure Blob/Static Website/CDN) with the right CORS/headers, keep your card at a Teams‑supported schema version (≤1.5, and try 1.4 if you see rendering issues), and avoid data URIs >25 KB.

What to do (practical checklist)

Serve images from a truly public, direct URL

For Azure Storage, prefer:

Static Website hosting: https://<account>.z13.web.core.windows.net/<path>/<file>.png

Or Blob public container with a direct blob URL: https://<account>.blob.core.windows.net/<container>/<file>.png (no redirects, no HTML landing pages). Test in an incognito window where you’re not signed in.

CORS and headers on the storage origin

Set CORS to allow Teams’ webviews to fetch the image:

Allowed origins: * (or the minimal set you require)

Methods: GET, HEAD, OPTIONS

Headers: *

Expose headers: (optional)

Max age: e.g., 3600

Ensure the blob serves the correct Content-Type (e.g., image/png) and avoid 302/307 redirects. This reduces intermittent desktop failures.

Use a stable, cacheable URL

Avoid short‑lived SAS query strings (or make them long‑lived) because Teams may cache placeholders if the URL 401/404s even once. If you must use SAS, ensure it’s valid well beyond the card’s lifetime. Some reports note that once a placeholder is cached in new Teams, it may persist until cache clear.

Keep the Adaptive Card schema version conservative

Set "version": "1.4" in your card JSON if you see rendering problems in Teams channels—even though 1.5 is “supported,” 1.4 can be more reliable across clients.

If you must embed (base64), stay under ~25 KB total

Data URIs work, but the total Teams message payload (including JSON) is ~25 KB, so images larger than icons/thumbnails will break. Better to host externally.

Try a CDN fronting your blobs

Putting Azure CDN (Standard/Microsoft) in front of blobs can normalize headers, reduce redirects, and improve reliability in the desktop client. Many teams report fewer intermittent failures with a CDN.

Test across clients + clear cache

Validate in: Copilot Studio test chat, Teams web, new Teams desktop (Windows/Mac), and mobile. If the desktop app shows placeholders, sign out/in or clear the Teams cache and retest. (There are known placeholder caching quirks.)