I would insert it into a custom fill pattern, which is a somewhat obscure
feature. With your drawing open, go to the View menu and choose Drawing
Explorer. This will bring up a tree view. Right-click on Fill Patterns in
the tree view and choose New Pattern. Name it (e.g., "wood grain") and click
OK. Then right-click on your new fill pattern in the tree view and choose
Edit Pattern Shape. Paste your wood grain bitmap onto the page that opens
and close and save it. Your custom wood grain pattern will now appear in
your list of patterns when you go to Format > Fill. You'll probably have to
experiment a bit to get it right. There is more information about this in
Help. Search for "custom pattern".