Only use one thumbnail, then follow it with empty a tags specifying the other larger images in their href attributes. Just make sure they all have the same bracketed group name in their rel attributes, ex:
HTML Code:
<a href="image-3.jpg" rel="lightbox[plants]" title="Click on the right side of the image to move forward."><img src="thumb-3.jpg" alt="Plants: image 1 0f 4 thumb" /></a>
<a href="image-4.jpg" rel="lightbox[plants]" title="Alternately you can press the right arrow key." ></a>
<a href="image-5.jpg" rel="lightbox[plants]" title="The script preloads the next image in the set as you're viewing."></a>
<a href="image-6.jpg" rel="lightbox[plants]" title="Click the X or anywhere outside the image to close"></a>
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