Copy all files to your CLOSURE_HOME/goog/editor/plugins directory.

Require the plugins:
goog.require('goog.editor.plugins.ImagePlugin');
goog.require('goog.editor.plugins.ImageDialogPlugin');


Then register the plugins:
    myField.registerPlugin(new goog.editor.plugins.ImagePlugin());
    myField.registerPlugin(new goog.editor.plugins.ImageDialogPlugin());

You can use the Image button from Google, or your own custom button. I use the Image button from Google.
    var buttons = [
      	goog.editor.Command.BOLD,
	    goog.editor.Command.ITALIC,
	    goog.editor.Command.UNDERLINE,
	    goog.editor.Command.FONT_COLOR,
	    goog.editor.Command.BACKGROUND_COLOR,
	    goog.editor.Command.FONT_FACE,
	    goog.editor.Command.FONT_SIZE,
	    goog.editor.Command.LINK,
	    goog.editor.Command.UNDO,
	    goog.editor.Command.REDO,
	    goog.editor.Command.UNORDERED_LIST,
	    goog.editor.Command.ORDERED_LIST,
	    goog.editor.Command.JUSTIFY_LEFT,
	    goog.editor.Command.JUSTIFY_CENTER,
	    goog.editor.Command.JUSTIFY_RIGHT,
	    goog.editor.Command.SUBSCRIPT,
	    goog.editor.Command.SUPERSCRIPT,
	    goog.editor.Command.STRIKE_THROUGH,
	    goog.editor.Command.IMAGE,
	    goog.editor.Command.REMOVE_FORMAT
    ];

If you are using the built-in Image button from Google, then make this change in CLOSURE_HOME/goog/editor/command.js:

goog.editor.Command = {
.
.
.
.
IMAGE: 'ImageDialog'
}

If the bubble is not removed on making the field uneditable, call this (by listening to goog.editor.Field.EventType.DELAYEDCHANGE/CHANGE):
		goog.editor.plugins.HelloWorld.removeAllBubbles(); 
to remove any instances.

TODO: Remove goog.ui.Bubble and use LinkBubble from closure-rev157.
Do not forget to calculate the dependencies.