Yes, plugin must work in editor (TinyMCE or FCKEditor). May be plugin was not correctly installed in your case. But problem is not is modrewrite or phpThumb - plugin even not works.
I reread your first post.... But nothing happens.. My source code doesn’s show a link.
I must explain how plugin works. It not changed any source code in your documents, It’s only generate new code on the site frontend or in manager editor in visual mode. But not change anything in source.
For exmple, if you have
<img src="assets/images/sample.jpg" width="100" height="100" />
in source of your document it will be something like this
<a class="thickbox" title="" href="images/12735ef262a71cfb4b1de760d2f66482/sample.jpg">
<img width="250" src="images/c0be78dc0fbeea794dc62cb8701f00c2/thumb_sample.jpg"/>
</a>
on the frontend and like
http://modx/image.php?&src=assets/images/sample.jpg&w=100&f=jpeg&q=75&sia=thumb_sample.jpg&hash=f5eec411487e0b43698be56948e72fc7
in the editor (small resampled image will be shown instead original big)
But when document will be saved, in source code still be
<img src="assets/images/sample.jpg" width="100" height="100" />