Similar to Smarthistory, Khan Academy has a huge range of resources that span across cultures and time.
Try searching in the section called Approaches to art history.
They are partnered with Smarthistory and Ted talks and many museums from around the world.
Art21 is a great resource that has short videos and essays about contemporary art. This website has series organised around themes like protest, structures, or memory. This is useful as it will give you some ideas about how to curate your own Comparative Study around a theme.
This is a useful resource for research as it summaries key ideas explored a particular artist, has useful information about cultural context and provides a useful description and analyses of key artworks made by the artists. This website has a good range of contemporary artists appropriate for Comparative study and links to additional websites for further research.
Please see your teacher for login details.
Art Through Time: A Global View examines themes connecting works of art created around the world in different eras. The thirteen-part series explores diverse cultural perspectives on shared human experiences.