In terms of picture quality and rarity of bicycles, it's tough to beat speedbicycles.ch. But Flickr has quite a bit to offer, too. Some of the groups I follow:

CLASSIC LIGHTWEIGHT STEEL BICYCLES (6200+ pics)

Campagnolo (3200+ pics)

Columbus Steel Bicycle Tubing (2800+ pics)

Vintage Italian Racing Bicycles (1000+ pics)

CLASSIC TIME TRIAL BICYCLES & STEEL TRACK BIKES (1100+ pics)

Bicycle Head Badge (3500+ pics)