I was browsing a bookstore (the old fashioned kind, with bricks and all) when I happened upon an interesting collection of royalty-free images by Dover Publications. Their online catalog shows the breadth of the selection available. Please note that the Dover site looks a lot like amazon.com but isn't.
Here's what I like about the series: The images are of high quality and come pre-scanned on a Windows/Mac readable CD-ROM. The images are lossless TIFF in AdobeRGB colorspace with Photoshop clipping paths included. The resolution of most of the images are typically higher than SL will allow which gives you the ability to work at high-resolution and scale down for import to SL. Finally, US$20 gets you a soft-bound book cataloging the images and the accompanying CD which yeilds a price of about US$0.20 per image. Royalty-free is nice as you need not worry about their use and sale in SL (or anywhere).
There is one caveat I should pass on: the images were apparently scanned from the Dover lithographs at 300dpi although the originals were printed with maybe a 260 linescreen process so they have the odd dustspeck on them and the printing dither pattern is visible in the supplied TIFFs. Both of these issues are readily correctable by someone skilled with Photoshop or PSP.
Disclaimer and Shameless Plug: I have no connection with Dover Publications. I have, however, released two new Art Nouveau stained glass pieces based on this product. They can be viewed and purchased at Malachi's Maladies in Miru or at SLBoutique.com. The first in the series (#113) is being sold for L$1 to introduce people to MP Glassworks.