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:
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Warning: file_get_contents(sites/all/themes/genesis/genesis/js/script.js) [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).
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Warning: file_get_contents(sites/all/themes/genesis/genesis/js/script.js) [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