The current drawer header does not really fit into the proposed dark theme. Using Konqui gives some nice branding. However I'm open for input.
I'm not a big fan of having Konqui there (or anything that's not an abstract image or a solid color). I don't have any arguments other than personal preference.
If you want to change it because the current image is from an old Plasma wallpaper, I would be happy to update it to the latest wallpaper as long as it's still abstract. Actually, my original idea was to change it with each release of Plasma (which I never did). Maybe we can make updating it a GCI task?
You put an image there for the sake of an image being there.
It is completely irrelevant to the menu.
How about putting there something relevant, like a png image of a phone and tablet and computer there showing connections?
I'm thinking on something loosely similar to these, maybe some KDE branding (a Konqi on the screen of the phone and pc display, etc.):
Discover gets a lot of criticism for the header image too. Same complaint: it's a pointless waste of space, an image there for the sake of being there. I really don't think this pattern works at all and would advocate removing it from Kirigami entirely. These images are just wasted space, and no matter how pretty they are, that won't change.