Quick reply...
In fact, I found them to be in THEME/calendar/qgn_calendar_bg_MONTH.
By default, they use .jpg file extension; the image will be read even if it is png-image with .jpg file extension.
"MONTH" is one of: apr, aug, dec, feb, jan, jul, jun, mar, may, nov, oct, sep.
Thank you.
By the way, Nokia is an excellent company; they created two beautiful themes, and about a dozen of image sets as continuous panorama-like-wallpapers. But they could not make several sets of twelve-months backgrounds for calendars, choosing by default the one which fits the user's geographical region. This eye-candy would have attracted much more users to Maemo's default calendar. But no, they simply put a dozen of black 800x424 backgrounds by default, giving neither instruction nor user-interface to change them.
Best wishes. Thank you, again.
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Per aspera ad astra...