Quite why you’d want to do this I don’t really know but there’s a new Google Calendar Labs setting that allows you to set a custom background (via URL) for your Google Calendar.
Now, I live and die by my calendar. It’s constantly updated, synced to my iPhone and everything but I just can’t see myself putting an image in its background. I think it would just make the calendar (mostly) too busy and hard to read.
More at the Google Tutor