You haven't said which version of Fireworks you are using. These
instructions are for FW MX.
Click on the bitmap thumbnail in the layers panel. It's important to
click on the thumbnail itself. Clicking on the sublayer isn't enough.
Once the thumbnail is selected, press CTRL (Control on Macintosh) and
move your cursor over the thumbnail of the path object in the Layers
panel. A cursor cue in the form of a tiny marquee should appear. When
you see the cursor cue, click on the thumbnail of the path object. A
marquee will appear around its edges.
Note that once ...