Aircraft spotters, older ones, who have scanned and uploaded their old pictures at places such as Airliners.net. Then there are two sites that scan old airline timetables. Great place to get clear images of the sort of winged logos that are found on the 1940s and 1950s airliners that are my speciality. Less useful are the two sites specialising in old airline logos - way too small to be of use.
Also very useful are people who kindly provide me with digital material from their private collections, such as El Al historian Marvin Goldman, Ansett historian Fred Niven and the Northwest Airlines Heritage Center.
So after a digital art/photo program such as Photoshop, the next most important thing is research.
David