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Whatever suits my mood at the moment. My buying preferences also tend to influence what my wallpapers are. Currently, I have several Call of Duty MW3 wallpapers I'm using now. Once the new iPad comes out, I'll probably have some Apple wallpapers then. As for a default, I tend to use nature/landscape pics. Basically a picture of some place other then where I'm currently at :eek:
 
Usually when each sports season comes around I choose my favorite team. That usually lasts for a month or so. Lately I have been choosing simple stuff. My current wallpaper, I got from somebody on here.

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I rotate about 20 of my own photos, mostly a mix of travel and London shots. Web-res versions of a few of the photos (the desktop versions obviously in 1920x1200 formats):

sanfrancisco-051.jpg

hollywood-malibu-05.jpg

washington-086.jpg

shanghai-part2-53.jpg

shanghai-28.jpg

london-skyline-night-2.jpg

london-skyline-night-1.jpg
 
I change it when I get bored with it. Roughly every other week.

As to what I choose: it can be scenery, it can be an abstract, it can be holiday-related (as it is now). Sometimes it's Apple-related; sometimes it's TV- or movie-related. For me, the overriding criteria is that it not be overly-detailed, particularly on the right edge where my desktop icons reside.
 
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