Guildhall Library's Print Room is one of the best sources of pictorial material on London history. Its collection includes tens of thousands of prints, drawings, watercolours, maps, photographs, and ephemera. The Print Room is now closed and the collection has moved to London Metropolitan Archives. The link above provides more information.

Guildhall Art Gallery is the home of the City of London's art collection. From its origins in the 17th century it has grown into one of the largest local authority collections, specialising in works of London interest and particularly strong in Victorian material. Located next to the historic Guildhall, the Gallery opened in its beautiful new premises in 1999.