I love using the Charlotte Mason method for picture study. It is so simple and yet, it makes such a profound impact on the students. My girls are teens, and we just recently started picture study. I have been using AmblesideOnline for our picture study, but recently discovered a very EASY and affordable way to to do picture study.
Simply Charlotte Mason has put together six wonderful artist studies. We just finished studying Giotto. It sparked some good dialogue and we learned about frescos, and learned to tell the difference between Byzantine and Medieval art techniques.
I highly recommend this curriculum, even if you do not homeschool. This is a simple way to introduce your children to art appreciation.