♻️ Consolidate external placeholder images to local examples/img (#25045)

[Rishi700252]

This PR is a first step toward consolidating external placeholder images (picsum.photos, placekitten.com) into locally-hosted images already available in examples/img, as tracked in #25045.

Changes

• examples/amp-story/helloworld.html: replaced picsum.photos image with local examples/img/sea@1x.jpg
• examples/amp-story/consent.html: replaced placekitten.com publisher logo with local examples/img/ampicon.png
• examples/amp-story/consent-geo.html: replaced placekitten.com publisher logo with local examples/img/ampicon.png

Verified locally with amp default that all three pages render correctly with the local images.

This is a partial fix — I plan to follow up with additional files referencing external placeholder services in subsequent PRs.

Addresses #25045

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