Add a checkerboard background for image preview
Image preview needs a background for better transparency support, e.g. white on white images are invisible in the Light theme.
Images with transparency can be difficult to view for various reasons and image editors tend to place a checkerboard pattern behind them to compensate.
I figured we could used a repeated image pattern for this then realised it wouldn't cover theming or dpi changes in the end user devices, so I mocked up something in CSS instead on the site directly:
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