docs: use the vision guide image in README example (#3849)

## Summary

The README vision example points to a Wikimedia thumbnail URL that can
fail during the API's image download. Replace it with the National
Gallery of Art image already used in the [OpenAI vision
guide](https://developers.openai.com/api/docs/guides/images-vision),
preserving the current Responses API example and model.

Fixes #2776.

## Image provenance

The image UUID `a2e6da57-3cd1-4235-b20e-95dcaefed6c8` is listed with
`openaccess=1` in the National Gallery's [official published-image
catalog](https://github.com/NationalGalleryOfArt/opendata/blob/main/data/published_images.csv).
The Gallery's [open-access
policy](https://www.nga.gov/terms-and-notices) releases these images
under CC0 for commercial and non-commercial use. The URL is hosted
directly by the Gallery and matches the official OpenAI docs example.

## Validation

- Ran the exact updated README URL example against the live Responses
API with SDK 3.13.0 and `gpt-5.5`: status `completed`, with a
description of the seated woman in the portrait.
- `git diff --check` passed.
- Reviewed the full diff: one image URL replacement in README.md.