What hints should UAs send when downloading images?

[eeeps]

This excellent question comes courtesy of @hamishwillee, who raised it in the discussions around #16.

My suspicion is, when downloading, end users want the largest-available image.

It doesn't really make sense to send Sec-CH-Viewport-Width in this case.

It may make sense to send Sec-CH-Width. Possibly some arbitrary/large value (requiring no spec changes), or some special keyword like canonical? (we'd have to spec this) Or maybe this signal belongs in some other header?