In structure->content type->yourcontenttype->manage display, set "Select the full image style" to the size you want (full image, etc), and set "Use jQuery modal for full image link " to "Colorbox".
======== Tried to do this point and got errors:
- *Warning*: file_get_contents(sites/all/themes/genesis/genesis/js/script.js) [ function.file-get-contentshttp://www.tony-mac.com/drupal/admin/structure/types/manage/gallery2/function.file-get-contents]: failed to open stream: No such file or directory in * _locale_parse_js_file()* (line *1393* of * /home3/tonymacc/public_html/drupal/includes/locale.inc*). - *Warning*: file_get_contents(sites/all/themes/genesis/genesis/js/script.js) [ function.file-get-contentshttp://www.tony-mac.com/drupal/admin/structure/types/manage/gallery2/function.file-get-contents]: failed to open stream: No such file or directory in * drupal_build_js_cache()* (line *4759* of * /home3/tonymacc/public_html/drupal/includes/common.inc*).
Hope this helps, Ursula