I do decoupage. If I were you I would color-copy the decal. If you stick the decal on (the “sticker” way) it might peel over time. Also, a decal is thicker than a color copy, so it will appear smoother and closer to the wood than an actual decal. Your goal is to have it last forever.
Color copy it, use Elmer’s glue, watered down to a milk-like consistency – about half water and half glue. Brush it on the wood, lay the color copy down, smooth it out, smooth out all air bubbles and make sure all edges are down. Let it dry completely, varnish. I use a water base poly, dries faster and is just as good as oil base. Oil base might make an oily mark on the paper (the paper of the color copy). Water base won’t. Which ever you choose, put about 5 layers of poly over the image. Let each coat dry before applying new coat.
What you’re doing sounds like a great project. Please post a photo when you’re done.