Making a decorative mirror is just a matter of using the terrain editor as a basis for the frame of the mirror. Simply create a grayscale image to use as the basis of a terrain with the resolution of 1024 x 1024 pixels. Then apply a wooden material to it.
Decide what kind of mood you want for your room before you make your image, notice the "bat-like" feel of the mirror frame to the right, this fits the mood for the room that I made.
If you use the gaussian blur in your image editor, you will get the appearance of beveled edges in your mirror frame.
Probably one of my favorite tips is the ability to make 3D gates, window panes, or stained glass frames -- with a symmetrical lattice! It's really easy and you get a GREAT effect.