How to backup large contact photos?
Backup and Restore

Following the app recommendations made in this XDA post How to backup local contacts with their high res pictures ?

I set a hi res 1984 × 1800 picture for contact and backed up using Super backup: SMS and contacts , took a backup , deleted the contact and restored

To my eyes, it appeared to be high resolution as originally​ set

The other app was not tried as it was pointed out in app review that it didn't work beyond 720 resolution

I tried to locate the storage of these high resolution pictures to manually verify the restored file size but couldn't. Method described in Where does android store contact photos ? didn't work



Edit


The reason I wanted to verify pre and post backup - restore process was to check if hi-res photos were backed up , not wanting to believe my eyes judging the resolution of the picture in a small circle on an LCD screen. So I asked this question High resolution contact pictures in Marshmallow - what is their size and where are they stored
As iBug explained in the accepted answer the resolution is capped to 720 by 720 irrespective of the picture size , an improvement implemented from KitKat days


It turns out that my suspicion was well founded !


As an aside, contact thumbnails (  used for notifications, messaging applications ) which are scaled down to 96 by 96 can be scaled up to 256 by 256 by this Xposed module Clarity


Bottom Line: Any contact picture irrespective of original resolution will look no better than a 720 by 720 resolution, whether you use an app , Gmail sync or for that matter in any manner