[roll=1]x[/roll] or [roll=2]x[/roll] or [nroll]xdn[/nroll]
Where x equals the number of die you want to roll. The =1 indicates 6 sided die and the =2 indicates the 12 sided die.
As for the nroll tag, that gives you a chart as opposed to images in case you want to do something besides the 6 or 12 sided die. I put 'xdn' in the tags. x indicates the number of die and n indicates the number of sides the die should have. The d should remain as a d.
Example:
Illegitimate Dice Roll!
Total: 11
Figured it might be useful for some forum games and whatnot.