First import your bitmap into the Flash Library, then look at the Color panel (make sure it's in Color Mixer), where it says type, choose Bitmap from the drop-down menu. When you do, all the bitmaps you have loaded into your Flash document will appear in the Color panel. Choose the one you want, then use the paint bucket to fill in the object.
To resize the bitmap after you've filled it in, use the Gradient Transformation Tool (in Flash 8 it is the icon on the second row, right side of the toolbar)