I would like to know where decoupage classes are offered in the GTA..
I'm sorry that no one has replied. I'm guessing that GTA is Greater Toronto?
I'm not from Toronto, so I can't give you specific suggestions, but you might want to ask at your local craft store (Michaels, Hobby Lobby and the like often have craft classes). If they don't have one scheduled, if they know there is interest, they may be able to set one up.
That said, you probably don't need to go to a class. Decoupage is pretty easy - apply some modpodge (glue) with a paint brush to the back of your image and the thing you want to stick it to. Set the image down, smooth it out and modpodge over the whole thing.
Here is a link that might be useful: craftfetish blog