The Image plugin allows one to have finer control over the layout of images than just the simple in-lining of images.
[{Image src='SandBox/test.png' caption='Testing Image' style='font-size: 120%; color: green;'}]
Shows the attachment SandBox/test.png with the caption Testing Image underneath it. The text is 120% in height and will be rendered in green color.
[{Image src='http://opi.yahoo.com/online?u=YahooUser1234&m=g&t=2' link='http://edit.yahoo.com/config/send_webmesg?.target=YahooUser1234&.src=pg'}]
Shows the Yahoo online / offline graphic and creates a link to Yahoo send message
While the ImagePluginWithThumbnails provides automatic creation of thumbnails from the full size image, it's also pretty complicated and not part of JSPWiki proper. But the Image plugin can do thumbnail links if you're willing to create the thumbnail images yourself, using the link attribute, e.g.,
[{Image src='images/ref/image_thumb.png' link='images/ref/image.png'}]
When you click on the image thumbnail the full size image will be displayed.