The MET is a fantastic resource for all things art throughout MANY different time periods. Side note, if you are ever able to go to NYC, you only have to pay $1.00 to get in! (Though the suggested donation is $10.00)
Browse the MoMA website for modern artworks.
Students can learn about contemporary artists with an in depth, mini biography about each artist. Brought to you by PBS.
NOTE: Most, but NOT ALL, artist are K-5 appropriate. Parents/Guardians should watch the videos beforehand.
Google has compiled images from museums all over the world so that people can take a look at paintings, see where they're located, and zoom in to see different textures. An AWESOME resource