Artstor is now a part of JSTOR database. You can access JSTOR here
You can use your Artstor login credentials to log in to JSTOR to have the functionality that you had in Arstor to make image groups and export them to PowerPoint and to be able to use other special features.
Here is a quick video on how to register for your JSTOR account if you don't already have an Artstor or JSTOR login, and how to use Artstor on JSTOR, how to use the presentation tool, make image groups in your workspace, and how to export to PowerPoint.