Thanks for that, I've just installed and enabled Fancybox and Shadowbox and went to view an image, and both of these plugins exhibit the same behaviour. Can the code in Joomgallery be adjusted so that it doesn't do this? I only viewed the centre image.
The attached example shows what's happening with Shadowbox.
EDIT: Highslide JS, DHTML Container, JavaScript Window and Thickbox 3 do not do this. Lightbox 2, Shadowbox, Slimbox and Prettyphoto do. Fancybox, when I click on an image, marks that image, the next one and the third images as viewed.