Because there are a lot of questions related to the image file format used by VirtualMirror here is a short answer…
At least a *.jpg of the frontal view of the eyeglasses is needed. This *.jpg should have a clean white background.
[at least needed]
Overlay, transparency, size, scaling, rotation, positioning will be computed by VirtualMirror itself.
Optionally VirtualMirror can use the Collada file format. This requires a web cam in any case. Both of the formats can be used together. VirtualMirror will try to enable the video mode if a Collada file (*.dae) was passed. Detailed information about Collada can be found at the following site: http://en.wikipedia.org/wiki/COLLADA