You can either use the img tag, or if you have the advanced editor turned on, you can just click on the image icon and add the URL.
Taking pics is easy - just line up the picture (either in edit mode or not) and use the Print Screen key - it's the default key for taking in-game shots.
Then, navigate to your screenshots folder (default location is C:/Program Files/City of Heroes/screenshots).
Posting them is tricky - you'll need a photo hosting web site like Flikr or Photobucket. In either case, once you have an account, you'll be able to upload photos to it - or screenshots in your case.
Once you upload the photos to your account you'll have a URL to share with others. either use the [url] or [img] tags on the forums to link or show the photo directly in your post, respectively.
How do you post pictures on here?