You might want to have her check out the Smithsonian American Art Museum http://americanart.si.edu/collections/search/
Another option is "ArtStor" http://www.artstor.org/index.shtml but it has a fairly high license fee. You can use it on a trial basis for 30 days...so it might be something to consider.
Hope this helps,
There are some resources for images in this ReadWriteThink.org lesson plan: http://www.readwritethink.org/classroom-resources/lesson-plans/picture-worth-thousand-words-116.html. The teacher might also get some additional ideas of what do with the art.
High res used to be difficult without paying for it, as Pati Teri notes with ArtStor or Davis http://www.davisart.com/Portal/DAI/DAIDefault.aspx.
However, you don't need art book publication resolution (350 dpi) if you're showing these digitally. You can get adequate image sharpness from around 144 mb if the screen is a regular classroom wall and that should work. JPEGs will deteriorate every time you move them, so download them fresh or if you want "archival" quality, go with TIFF.
Having said all that, the Picturing America website, which is part of EDSITEment, can offer you more pretty good resolution sizes for 44 images in a PP presentation that you can download and then open at http://picturingamerica.neh.gov/media.php?subPage=media_images
Further ways to access good individual images are through major museums such as the Prado site, which is fantastic and the Rijksmuseum. That's just a start. Look at the Louvre, which is not bad. More will follow as technology improves. And don't forget the Google Art project, which now has apps as well.
If these are for classroom use, fair use should cover it. However, publication on the Web or other venues will get you into copyright problems for the digital photo you use from the above sources.
Hope that helps.
Here are a few collections that may be helpful:
Have you seen the National Gallery of Art website that provides 25,000 free images of art that may be useful in art history classes? The site offers famous and not-so-famous art. Nearly all of the images can be downloaded and re-used for free. NGA Images also allows you to register and create online collections of images called lightboxes.
In addition, The Metropolitan Museum of Art hosts 372 art history books online. All of the books can be read online or downloaded as PDFs (some are huge files).