Attach one or more images (with alt text and aspect ratio) to a Bluesky post by uploading each image as a blob and embedding it via app.bsky.embed.images.

Documented steps

1. Read the image file into memory and check its size before uploading; the app.bsky.embed.images Lexicon schema caps each image blob at 2,000,000 bytes (2 MB) - reject or resize larger files client-side before calling uploadBlob.
2. POST $PDSHOST/xrpc/com.atproto.repo.uploadBlob with header Content-Type set to the image's exact MIME type (e.g. image/png, image/jpeg, image/webp), Authorization: Bearer <accessJwt>, and the raw image bytes as the request body (not JSON, not base64 - the request content type is '*/*' per the API reference, meaning raw binary).
3. The response is {"blob": {"$type": "blob", "ref": {"$link": "<cid>"}, "mimeType": "image/png", "size": 760898}} - keep this whole object, it is embedded as-is (not re-encoded) in the post record.
4. Determine the image's pixel width/height (e.g. with Pillow in Python) to build an aspectRatio object {"width": <int>=1, "height": <int>=1}; if you cannot determine the true aspect ratio, leave aspectRatio undefined rather than guessing, since an incorrect ratio degrades client rendering.
5. Build an image entry: {"alt": "brief alt text description", "image": <blob object from step 3>, "aspectRatio": {"width": 1280, "height": 760}}. Both 'image' and 'alt' are required fields on each entry - alt text cannot be omitted even if empty-string.
6. Repeat steps 1-5 for up to 4 images total (app.bsky.embed.images.images has maxItems: 4), then set the post record's embed field: {"$type": "app.bsky.embed.images", "images": [ ...entries... ]}.
7. createRecord the post as usual with collection app.bsky.feed.post, including 'text', 'createdAt', and the 'embed' field built above.