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Left and right arrows do what you think. The middle oval will switch the images to an higher resolution. They will load more slowly. The oval will also set the images back to a lower resolution if the current image is high resolution.
At high resolution repeated clicking before a picture loads will cause the browser to give up. You will need to start over.
You can download images you want by right clicking and selecting save image as (Not quite sure how you do it with a Mac but you probably do).