This is probably a well known site to all else but I just came across a part of the advrider.com forum that is an absolute repository of vintage dirt photographs, etc. Look at Advrider > Bikes > Old's Cool for 210 pages of some great stuff. Page 116 has a cool Penton motocross shot for example.
Forgot to add topic Dirt Bike History 101 - this is the one with the 210 pages of good vintage related postings.