Plugins are very easy to make and register. Here's a simple one that does nothing but support installation and uninstallation. All plugins need to do at least this much.

public class SimplePlugin: IPlugin{
    public IPlugin Install(Configuration.Config c) {
        c.Plugins.add_plugin(this);
        return this;
    }

    public bool Uninstall(Configuration.Config c) {
        c.Plugins.remove_plugin(this);
        return true; //True for successfull uninstallation, false if we couldn't do a clean job of it.
    }
}

You can install your own plugin just like any other - make sure it's referenced in the project, then put <add name="Simple"/> into the <plugins> section of web.config.

Or, you can install it during application start using new SimplePlugin().Install(Config.Current);, just like any other plugin.

If you aren't following the convention of naming your plugin ImageResizer.Plugins.Simple.SimplePlugin, you'll need to specify the fully qualified name in web.config, i.e. MyNamespace.MySubNamespace.MyPlugin.

See the Gradient plugin for an example of a short, yet very capable plugin.