The Tweaker shows the Description dialog with the description of the selected variable.
If you want to enhance the description by adding a picture to the dialog, place a picture by using following rules:

Rules:
FILENAME:		The name of the cvar.
FILE FORMAT: 		JPEG (*.jpg)
RECOMMENDED DIMENSIONS:	400x300

So if you want to add a picture to the 'r_shadows' cvar, you should place an image with 'r_shadows.jpg' filename.

Note: the tweaker will stretch the image (to 400x300).