Note 1: The window needs to resize, or you need to switch between tabs
and subwindows for the change to be activated without a restart of Krita.
This is a Qt issue, so we cannot do anything about it.

Note 2: If you're feeling in a weird mood, manually edit the kritarc file
and add

mdiBackgroundImage=/path/to/an/image/that/Qt/can/load

And you'll get that tiled as your background. Ever wanted to draw on a
nice faux leatherette blotter? As if it's 2000 and Apple wants its
skeuphoemorphics back? This is your chance :-)

BUG:342549
BUG:342161