BibliOdyssey points out that the British Museum has made a large proportion of its collection of prints available online:
The size of the database is enormous. There are more than 13,000 satirical prints for instance. A free text search on ‘London’ produces a similar number. There are over one thousand prints by Albrecht Dürer. ‘Ornament’ returns more than three thousand images. Although the image sizes vary, most are at least close to screen size and there is no watermarking.
A wonderful resource.