Go to your Joomla backend -> components -> CComment -> settings -> new, from the component dropdown field select com_joomgallery and click next. Save the configuration.
Now go to Extensions -> Plugin Manager and enable the Joomagallery - CComment plugin. Now in the frontend of your site under every image you should see the comment form & in category view you should see a comment counter under each image.
If you don't like the position at which the comment form is included you'll need to edit components/com_joomgallery/views/detail/tmpl/default.php and move line 288
to a different location.