To download images from Artstor you will need to create a personal sub-account. Here are the instructions for registering for an account.
On the upper right corner of the next screen, select "Login or register...".
Follow the instructions to set up your new account (it's free for registered MiraCosta students), then log in. Make sure pop-ups are enabled on your browser (use Chrome or Firefox).
Once you are logged in, follow these directions for downloading an image.