Ma Wing Pui;
You can alter the transparency of a jpeg image that is
into fireworks, however no image editor can save or export a
jpeg image with
transparency because the jpeg format simply does not support
Depending on your needs, you might be able to achieve the
effect you need with an opaque .jpg.
If the .jpg is to go over a solid color, modify the canvas
color to your background, set the opacity of the .jpg image to 50%,
and export. The resulting .jpg won't be transparent, but it will
have that effect.
If the .jpg is to go over other content in a fixed position,
then import the other content in the proper placement. Layer the
.jpg over it, set the opacity to 50% and export. Again, the result
is not a transparent .jpg, but you get the effect.