You add [img]url_to_image_here[/img] in the same place as your current signature.
(click user control panel, click profile tab, click edit signature)
to add in game text: take a screenshot, edit the screenshot with an image editor (crop/cut out unwanted parts and resize) to get the text you want, upload to an image hosting site (imgur, gyazo, etc.), take the url of the uploaded image and put in your signature with the above code.
the rank bar is from Ric's UserBars thread (
http://kreatious.net/viewtopic.php?f=3&t=93).
other pics work the same way. get their url and add it to your sig with the code.
you only have so many characters to work with for a signature, so you may need to shorten the url's with
http://www.tinyurl.com