*Existing copyright law (in the US) can be interpreted, as pjz says, to state that you cannot copy a gallery image to your hard drive.

On the other hand, as someone else pointed out, your browser automatically makes a copy of all images so that they can be viewed. So it could be argued that, by uploading an image to the gallery, the artist gives implicit consent for the viewer to download the image for viewing purposes only.

*The main difference in the two is the intent, which I think can be argued pretty easily.

Image being downloaded to the internet temporary files is a process inherent to the act of viewing an online image, and internet temporary files are subject to periodic purging, without being used elsewhere.

Act of saving one's image on a hard drive by deliberately clicking 'save image as' or similar, like drag drop etc... with the intent to save and use personally in places other then temporary internet files, and with purposes other then viewing online within the context of a gallery.
Desktops, skins, screensavers etc....

Yes, the author of an image DOES have the right to define in which manner their images or other copyrighted property is to be used. Much like merchants and freebie providers being able to specify whether their product can or can not be used commercially.

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