I know that Mariner 6 & 7 were tracked for a considerable period after the Mars encounter and that some additional (engineering related?) imagery was taken.

However this is a period that does not get covered in the more general discussions I've read of the mission which effectively end after the Mars encounter.

Does anyone know just how long contact was maintained and if those post encounter images are available in some form?

Contact with Mariner 6 and 7 was maintained for almost 2 years after launch but the exact dates of their last communication do not appear to have been published (ref. 'Robotic Exploration of the Solar System', Ulivi/Harland, page 87). Mariner 6 collected about 10.5 hours of ultra violet spectra from the milky way, and between april and mid may 1970 both spacecraft were on the far side of the sun, as seen from Earth, and used to measure the deflection of their radio carriers caused by the sun.

As far as I know their camera systems were no more used after the Mars flyby (in contrast to Mariner 4 which took some calibration images after Mars).

As far as I know, Geert is right. I have never heard of Mariner 6 or 7 taking post encounter images.

I can't find anywhere detailed information about the cameras of Mariner 4/6/7... Are they phototelevision ones/like Zufar?

I've been doing some hunting on the NTRS to see if I could find answers, and I think I may have answers to both the question in the OP and this one.

According to the Mars '69 Press kit (Link) both cameras were photo-television units, one camera ('A') was essentially the same as the Mariner 4 camera aside from being fitted with a wide-angle lens. The second camera ('B') was the narrow angle unit, but it's not clear if it was otherwise identical to the 'A' camera.

The other document I found is volume 3 of the JPL tracking report (Link), which covers the extended mission. It would look like last contact with Mariner 6 was around the 21st of December 1970, with the last contact of Mariner 7 being around 28th December 1970 at which point it had run out of attitude control fuel (They are the last dates mentioned).

No mention of imaging is made but there is reference to a final engineering experiment involving firing the course correction motor and using the UV spectrometer to study the exhaust plume, it's not clear from the document if this was done with both spacecraft or just Mariner 6.

If the cameras were indeed phototelevision units, then imaging after the primary mission should have been impossible.

They were slow-scan vidicon. Mariner 9 and 10, Viking 1 and 2, and the Voyagers also used it. No film was involved.

I resurrect this topic to share something I only recently found that finally solves what for me had always been a small mystery.
you may remember in fact that the dates of the last contact with Mariner 6 and 7 are apparently not available. I knew that there had been solar conjunctions observed in spring 1970, but how long again the probes were heard was not clear. I have finally found the information in the 3rd volume of the technical memo JPL-TM-33-474 "Tracking and data system support for the Mariner Mars 1969 mission. Volume 3: Extended operations mission"

the extended mission was funded up to 30 December 1970, and it appears that telemetry was last received from both spacecraft on DOY355 (21 December 1970). Both spacecraft were to be placed "in final state" on 23,29,30 December.
Note also on page 70 this sentence: