this might help but....
try
www.michaels.com
It's a craft store (or at least it should be on the website.) Anyway, they list tips and project sheets online. Decoupage should be one of them.
Anyway, I have heard to start with a clean surface so i would assume unfinished. And you can do magazine pictures cuz I have seen it done so I think photographs and printer paper will work. They sell the glue stuff at craft stores.
ETA: I agree though with the above post... if you use an inkjet, the ink will probably run when wet. See if you can get a sealent or print on a color laser printer... maybe.