To change the email address/any text from text into image. This can be done automatically in Drupal using module called Textimage. I tried installing it and the result looks good. (REF: http://drupal.org/project/textimage)

The image background and font size/family can be set through this module. Auto-insertion can be done when buddled with this module. (REF: http://drupal.org/project/textimage_autoinsert)