Because Flash is a vector based program, imported images need to be vector images to be usable in the same manner that images made directly in Flash are. Vector images are far easier to manipulate and alter, and they can take advantage of all of the neat tweening that Flash can do.
Given their usefulness, Flash provides an easy method for converting bitmap images into vector images. To convert the bitmap image into a vector image, click on File--Import in Flash. From the window, choose to import the bitmap image. Once you have the bitmap image open in Flash, click on Modify--Trace Bitmap. A new dialog box offers you several options for controlling the look of the vector image. Change these settings (or test them out and use Edit--Undo to go back to the bitmap version and try again with new settings) and then your bitmap image will be a vector image.