The people at oldbookart.com are living the dream.  They sit around all day scanning the most beautiful illustrations from the most beautiful old books, which they then post on the internet for free.  They'll also put them on neck ties and coffee mugs if you want them to.  All of the images are from the 19th century or earlier and so are fully in the public domain.  The ones below are just a tiny sample, being just the first ones I came across and not even the best of the lot.  They claim to have scanned over 3200 images so far, with more being added all the time, and most are pretty high resolution.