I needed a simple plugin for TinyMCE, but it seemed impossible to find any kind of real examples. All that I needed was a custom button, and when that button was clicked, an image to be inserted into the text. No popups, nothing fancy at all. I'm using TinyMCE version 2.0.9 for this, and in the end I integrate it into Drupal 4.7's TinyMCE module. This should work without the Drupal integration, though.
Here's an example of creating such a plugin. We'll just call it icon, to be generic.