UPDATE: This post is quite old now and wont work with the latest version of WYSIWYG API, will try to update this soon.
Gallery2 is one of the best galleries available which could be integrated with Drupal easily you would have read my post on integrating it earlier, after you have integrated it the bigger question is how get those images into your content or nodes so this posts answers it, after reading this post you would be able to integrate Gallery2 with your WYSIWYG (TinyMCE) Editor.
'G2Image' => array(
'path' => $editor['library path'] .'/plugins/G2Image',
'buttons' => array('G2Image' => t('G2Image')),
'internal' => TRUE,
'load' => TRUE,