What visual artifacts are expected from the JWST?
Are you asking about the PSF (point-spread function)?

There are some simulations at http://www.stsci.edu/jwst/software/webbpsf ; there are some basic images available at that site as well as a downloadable package you can use to compute the PSF for a particular instrument and wavelength.  Since the telescope hasn't been fully assembled yet these are based on simulations.