They have a huge number of works online from just about every artist I’ve ever heard of. Not only can you view very nice, full-color files of the major masterpieces, you can see many minor sketches and studies by the artists. (You can also copy the files from the site — very unusual for an art website.)
This painting is by Giovanni Boldini of Consuelo Vanderbilt, the Duchess of Marlborough, and her son, Ivor. Consuelo has a fascinating history that I am currently researching for a possible writing project. I loved this painting of her and that lead me to look at, and be delighted by, some of Boldini’s other works on ARC.
If you’re an art fan (or aspire to be one), you might enjoy the site, too.