Led picture frames?

Hello i have etched glass and now i want to put leds in the background of the piocture frames. I need to know how to hook the leds circuit up to a normal plug in, so that they dont have to run off batteries. i have an ac ac adaptor it says direct plug in class 2 transformer model 28 – a09- 200, input ac 120v 60Hz 3 w , output ac 9V 200mA and it looks like this (but is not that one) http://www.montek.com/cart/files/thumbs/… so i think the out put would be 9v right so what kind of resitor would i need for my project it will have like almost 20 leds all hooked together so any help would be great and could i just have one resitor then all the other leds hook in seriers (or paralle) this is to be plugged into a normal wall outlet