Are there any Hubble Telescope pictures of quasars or have they only been detected by radio telescopes?
Yes, Hubble has taken pictures of quasars.
See the Hubble site http://hubblesite.org/gallery/album/galaxy/quasar_active_nucleus/ - not all of the photographs are quasars, but their are a few.
Remember that Hubble takes greyscale images at a number of wavelengths. Not all of the wavelengths are in the visible spectrum. The images are combined and given visible colours. So, most Hubble images are not in true colour.