Change image dimensions and source in README

[Jun Yan (junyan72)]

Updated image source and dimensions in README.

Summary

Validation

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

[Copilot]

Pull request overview

Updates the README’s demo table to point the “Image” example at a new GitHub user-attachments asset and adjust its displayed width.

Changes:

• Updated the “Image” demo <img> src URL.
• Adjusted the image width from 400 to 361.
• Tweaked the image alt text.