I have Lightbox2 installed on the site which means you can use it in your posts for thumbnails if you want.

here's an example

First of all if you don't know how to post images read this. How to Post Images.

When you are uploading the image be sure to check the Box next to "Creae Thumbnail."

It will tell you thumbnail created. By default it adds "_0" to the file name of the thumbnail. Click add for the thumbnail's image. When you are in the image insert window copy the file URL.

Now click the image in the editor then click the link button.

paste the images url into the Link URL text box and remove the "_0" from the file name.

now click the html button

Type rel="lightbox" somewhere within the <a href="..."> tag so that it is something like this
<a href="blah.jpg" rel="lightbox"><img src="blah_0.jpg"></a>

Now click Preview and see if it works. If so congratulations you have a sweet thumbnail for your post. Do it over for more thumbnails. If you did something wrong go back and try to fix it or email me. betaswap@gmail.com