I’m a content/project manager, and we recently launched a web redesign with an image-heavy front page. Something we discovered was that our low-res JPGs were still retaining too much information and slowing our load times well below what we were comfortable with for the user experience. Our team recommends, as much as possible, using PNG format as a result. We find it renders attractively at an extremely compressed size.
How does this compare to libraries? The first thing to ask is the purpose of your archive. Is it to serve as an online gallery? In that case, I think something like PNG or GIF would be best. Obviously, a digital library is more than just a gallery of images, but if a user has to wait 10 minutes for it to load, they’re not going to wait around to dig into your spectacular metadata.
That said, if there’s an intent for use offline—you want visitors to download something or expect that certain documents will be printed—you may want to offer a higher-resolution version, or at least an option to download a higher-resolution version (Personally I’d go JPG over TIFF, as most free photo viewers can open a JPG but not all can open a TIFF).
I think it’s important to remember that technology evolves. I found Rick Matthews’ treatise on image types incredibly informative (I honestly didn’t know that BMPs were Microsoft-proprietary materal), but I can now include transparency on websafe GIFs when I’m saving them in the newest version of Photoshop.