Resize examples screenshots

The old ones were 1600x1200, resized in the HTML to 800x600 (ratio 1.33).

The new ones after #24898 are 1440x900 so with a ratio of 1.6, yet we still resize them in the HTML to 800x600 (1.33).

IMO, we should resize the PNGs to whatever dimensions we show the thumbnails. Sure, they won't look as crisp as they can on retina displays but it's so many bytes to ignore.

/CC @mdo: ping me if you resize them and make a PR so that I can compress them before we merge.