Hi pablitinho,
The ultrasound simulation examples in the toolbox are in 3D, as this allows the properties of a real ultrasound scanner to be captured (for example, wave propagation and scattering in 3D, the finite size of the detector elements in the elevation direction, etc). One option would be to stack your 2D image into a 3D image, adding some Gaussian noise to mimic diffusive scatterers. Alternatively, you could replicate the functionality of the 3D code in 2D, being mindful of the assumptions you are making in doing so (this will require a bit more coding).
If you are just interested in producing something that looks a bit like an ultrasound image (and is computationally efficient), you could also take a look at ray-tracing methods, e.g., Shams, "Real-time simulation of medical ultrasound from CT images", 2008.
Hope that helps!
Brad