Below are the steps to change the "NewTopic" button to a "NewIssue" one from the phpBB bugtracker website in the SubSilver2 style:

Step 1:

File: ROOT/tracker.php

Find:
  'TICKET_IMG'     => $user->img('button_topic_new', $user->lang['TRACKER_POST_TICKET']),

Change to (Or Replace With):
  'TICKET_IMG'     => $user->img('button_ticket_new', $user->lang['TRACKER_POST_TICKET']),

Step 2:

File: ROOT/styles/subsilver2/imageset/your language/imageset.cfg

Add:
# Tracker images
img_button_ticket_new = button_ticket_new.gif

Step 3:

File: ROOT/includes/acp/acp_styles.php

Find:
   'buttons' => array(
    'icon_back_top', 'icon_contact_aim', 'icon_contact_email', 'icon_contact_icq', 'icon_contact_jabber', 'icon_contact_msnm', 'icon_contact_pm', 'icon_contact_yahoo', 'icon_contact_www', 'icon_post_delete', 'icon_post_edit', 'icon_post_info', 'icon_post_quote', 'icon_post_report', 'icon_user_online', 'icon_user_offline', 'icon_user_profile', 'icon_user_search', 'icon_user_warn', 'button_pm_forward', 'button_pm_new', 'button_pm_reply', 'button_topic_locked', 'button_topic_new', 'button_topic_reply',
   ),

Change to (Or Replace With):
   'buttons' => array(
    'icon_back_top', 'icon_contact_aim', 'icon_contact_email', 'icon_contact_icq', 'icon_contact_jabber', 'icon_contact_msnm', 'icon_contact_pm', 'icon_contact_yahoo', 'icon_contact_www', 'icon_post_delete', 'icon_post_edit', 'icon_post_info', 'icon_post_quote', 'icon_post_report', 'icon_user_online', 'icon_user_offline', 'icon_user_profile', 'icon_user_search', 'icon_user_warn', 'button_pm_forward', 'button_pm_new', 'button_pm_reply', 'button_topic_locked', 'button_topic_new', 'button_topic_reply', 'button_ticket_new',
   ),

Step 4:

Add to the database (REPLACE phpbbmod WITH YOUR TABLE NAME, AND CHANGE phpbb_ IF YOU HAVE CHANGED THE DATABASE PREFIX FROM THE DEFAULT.  ALSO CHANGE THE IMAGESET ID TO SUIT THE TEMPLATE YOU ARE USING, WHICH YOU CAN SEE IF YOU HOVER OVER THE DETAILS LINK IN THE ADMIN/STYLES AREA IN THE STATUS BAR LIKE &ID=2 ON THE END. ASWELL AS YOUR LANGUAGE SHOWN HERE AS EN FOR ENGLISH):

INSERT INTO `phpbbmod`.`phpbb_styles_imageset_data` (`image_id`, `image_name`, `image_filename`, `image_lang`, `image_height`, `image_width`, `imageset_id`) VALUES (NULL, 'button_ticket_new', 'button_ticket_new.gif', 'en', '0', '0', '2');

Step 5:

Add the button called "button_ticket_new.gif" to the imageset/your language/ folder.

Step 6:

Refresh the styles template, theme & imageset in the admin/styles area and clear the cache using the button on the admin home page.