Apparently, there is a problem with Java and ARTstor. Here's the solution (from a company message):
A number of ARTstor subscribers have contacted us recently to report that their campuses have disabled Java software on their Web browsers due to security concerns, which interferes with downloading individual images from the ARTstor Digital Library. Oracle, the company that created Java, has released a new version of the program that may resolve these issues, and it can be downloaded from their website: http://java.com
Meanwhile, we are currently working on a new solution that will not use Java. Until then, we urge users who do not want to continue using Java to use our PowerPoint download instead. You can learn how here. We will announce the details of our new solution in the coming weeks. In the meantime, we apologize for any inconvenience.