Good news, everyone: today (and rather late) I've discovered that KuickShow has indeed been ported to KDE4, contrary to initial rumours and creeping suspicions. Aaaah, that's so much better! Gwenview might be interesting for users who only want a red eye correction, but it certainly doesn't meet my needs.
And my needs are truly very humble. I want the image to open immediately and without any borders, menus or further ado (in KDE, get rid of the borders via 'Advanced/Special Application Settings'). I also want to be able to rapidly adjust brightness, contrast and gamma of a given picture, which in kuickshow is as easy as pressing b, c, and g (plus shift to reverse the action).
You should see the flabbergasted Windows users when the image magically transforms in front of their eyes without me performing any mouse acrobatics. :D