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:
Boeing's photos are smaller, but they are quite good:
Embraer's site is very similar to Cessna's, with some very nice photos:
Bombardier has a lot of air-to-air photos of their line, including the Dash-8, CRJs, and Lears:
Mooney. You feel like your spending a fortune just looking at them. Photos are quite small, though:
Beech/Raytheon. These photos are very nice, like the rest. Nice light and compostition. You need to select the aircraft type, then select Photos:
Bell-Textron. They make good helicopters. They also post small photos. Nothing special here, but:
If you know of more, please post them!
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: