To select an image, use the checkbox on the upper right. The selected image will be framed in gray. To select a sequence of images, you can use the checkboxes or just click within the boundary of the image you're selecting; if you do the latter, the image you selected most recently will be framed in blue. You can select all the images on a page by choosing "Select all on page" from the "Organize" menu at the top left.
To deselect an image, clear its checkbox or click within its frame. To clear all your selections, click on the bold red statement about the number of items selected at the top, or choose “Deselect” from the "Organize" menu at the top left.
You can add individual images to a group by clicking on the "ADD TO GROUP" button. This allows you to add the item or a detail of it to the group of your choosing, either new or existing. You can save multiple selected images on a page to a new or existing group by choosing one of these options under the Artstor Organize menu.
For new groups, enter a group title. You can add a description and descriptive tags (these are searchable, so they will help you to find your group in the future; search for groups by title or tag in the Groups area under the Artstor Browse menu).
More on Groups in Artstor (from the Artstor Support site)