chore: consolidate image serving for examples and tests

[rajanarahul93]

✨ [Examples] Consolidate image serving in examples and tests

Description

This pull request consolidates image serving for AMPHTML examples and test files, addressing issue #25045.

• Why:

  • Ensures all image references in examples and tests point to local assets within the repository, improving reliability, reproducibility, and self-containment of AMPHTML development and demos.

• What:

  • Replaced all external image URLs (Picsum, Unsplash, Googleusercontent, amp.dev) in example HTML files with local images from examples/img.
  • Updated examples/images.json to reference only included images.
  • Verified that all examples use repo-hosted images for static demonstrations.

• How to verify:

  • Open affected example files and confirm that all images render correctly.
  • Check that no image references use external URLs.

Closes #25045

[CLAassistant]

All committers have signed the CLA.

[rajanarahul93]

Hi @cathyxz, could you please let me know if any further changes or updates are needed on this PR before it can be merged?