Visual Resources for Eighteenth Century Art -- Any suggestions please?

Hi there,

I was just wondering if anyone might be able to suggest any reputable online visual resources (preferably those specific to eighteenth century Art)?

Aside from ARTstor, the Getty Research Institute and various National Gallery sites, I'm only finding erratic (and sometimes questionable) websites that contain eighteenth century Art images.

I am writing a paper at present and I'm just trying to get a feel for online visual research tools in Art History (preferably open-source). For example, those that display high resolution images with the facility for zooming in and/or comparing two images at once, etc.
(National Gallery, London: http://www.nationalgallery.org.uk/paintings/after-anthony-van-dyck-portr...)

Any suggestions, advice or thoughts would be very much appreciated.

All the best,

Neassa