Hi there --
there are 2 ways.... to do it.. give a callback and generate your own code for the image (that might just be nasty)...
else - extract the feature from this library....
https://github.com/blakdronzer/POWER-GCrud
u may ignore all other stuff.. but take up the following
application/libraries/grocery_CRUD.php
application/libraries/img.php
application/config/img.php
application/config/grocery_crud.php
and make a provision for the thumbnails being cached in assets/grocery_crud/thumbcache
basically this GC library is modified to provide a cached thumbnails generated for bigger images like this and render automatiaclly @time of listing / edit / read - thumbnail for the same and on click - it will enlarge - as expected.
Happy GCing..:)