The best way to have students view images is to link directly to an image in Artstor using the supplied persistent link. That being said, Artstor does allow you to download and reuse images in Canvas. Images cannot be reused in publicly accessible webpages, as this violates Artstor's Terms and Conditions of Use (as does any non-educational use).
Be careful when reusing images from the Shared Shelf Commons in Artstor (you will see an S directly underneath these images in the results screen, as opposed to an A which indicates an image is in the main collection); these are images that are uploaded by various institutions, so it is impossible to make blanket statements about how they can and cannot be reused. Rights information is included in the records of the items in the Shared Shelf Collections.
Artstor supplies the HTML embed code needed for a linked thumbnail image (see below example).
To access this feature, try clicking on an item's description in the results screen (directly beneath the thumbnail). This will bring up the item record. Next, select the 'File Properties' tab and you will see the HTML for the linked thumbnail image in the 'Embedded Image URL' field.
Here are some freely available resources that provide reusable images: