You don't apply a background image to the canvas. The canvas can be transparent or a color.
What you can do, however, is place a bitmap as the lowest element in the layers. For example, This Fireworks .png document has three vector elements on a transparent canvas, but I put a bitmap below them:
Here's a view of the Layers panel, showing the bitmap at the bottom of the stack:
If you have a small image that you want to tile across your entire document, place a rectangle at the bottom of your layer stack, making it the same dimensions as your entire document. Then, you'll need to define a new custome texture and apply that as a fill to your rectangle. For instructions, check the Fireworks Help on textures.