While the pics here at airliners.net are very good, the manufacturers have posted many professional photos of their products which make excellent wallpapers.

For those who haven't seen it yet, Cessna has an amazing gallery of very beautiful photos of all their aircraft. This is my favorite gallery of them all:
http://www.cessna.com/aircraft/

Boeing's photos are smaller, but they are quite good:
http://www.boeing.com/companyoffices/gallery/flash.html

Embraer's site is very similar to Cessna's, with some very nice photos:
http://www.embraer.com/english/content/aeronaves

Bombardier has a lot of air-to-air photos of their line, including the Dash-8, CRJs, and Lears:
http://www.bombardier.com/en/3_0/3_0_1_6_4.html

Mooney. You feel like your spending a fortune just looking at them. Photos are quite small, though:
http://www.mooney.com/company/photos.htm

Beech/Raytheon. These photos are very nice, like the rest. Nice light and compostition. You need to select the aircraft type, then select Photos:
http://www.raytheonaircraft.com/

Bell-Textron. They make good helicopters. They also post small photos. Nothing special here, but:
http://www.bellhelicopter.textron.com/encyclopedia/photos.html

If you know of more, please post them!

-Squigee


Oh yeah, there's a company which makes flying busses. They have very few photos (at least that I can see) but there are a few posted with the news releases:
http://www.airbus.com/media/media.asp