[docs example confusion report] Dark vs. light image examples shouldn't look the same

[pradyunsg]

Discussed in #835

^{Originally posted by webknjaz September 3, 2024}
I was just showing https://pradyunsg.me/furo/reference/images/#different-images-for-dark-light-mode to a tech writer and wanted to demonstrate that toggling the mode at the top of that web page changes images.
I was expecting that the difference would be visible, but the images looked the same.
It took a minute to figure out the confusion — the image URLs are the same and the image is not different in any of the variants of this example.

I propose switching the example image to be different for dark vs. light modes. And additionally, it'd be good to use different images for image vs. figure.

[pradyunsg]

A PR fixing this would be welcome.