First you must activate the fileupload module in DG3 Application Settings.

Here you define the default upload area for the images and the auto generated thumbnail images used to select images from the server in the richtext editor. The images can only be uploaded to an area in the static/ catalog in dg3, so if you set Default image path to ulimages and Default thumb nail path to ultumbimages, you will be able to refer to a image using /static/ulimages/imagename.png as the src.

Then you add a richtext field to you page and add the following urls:

Image browse url: /_upload/redactorimagebrowse/

Image upload url: /_upload/redactorimage/