Airliners.net is a good place to go for photos of various aircraft types and airline livery.

Just to add:
737-200 - old cigar-shaped turbojet engines
737-300+ - CFM-56 family turbofan engines
747-200 - short upper deck with 3 windows
747-300 - extended upper deck
747-400 - 6 ft winglets
777-200+ - three-axle main landing gear

[This message has been edited by Indurain (edited 01-02-2001).]