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

Step 1:

File: ROOT/styles/prosilver/template/tracker/tracker_tickets_body.html

Find:
<div class="post-icon"><a href="/{U_POST_NEW_TICKET}" mce_href="/{U_POST_NEW_TICKET}" title="{L_TRACKER_POST_TICKET}"><span></span>{L_TRACKER_POST_TICKET}</a></div>

Change to (Or Replace With):
<div class="ticket-icon"><a href="/{U_POST_NEW_TICKET}" mce_href="/{U_POST_NEW_TICKET}" title="{L_TRACKER_POST_TICKET}"><span></span>{L_TRACKER_POST_TICKET}</a></div>

Step 2:

File: CSS

Add:
 .ticket-icon span {
      
         background-image: url("{IMG_BUTTON_TICKET_NEW_SRC}");

}

 .buttons div.ticket-icon {
         width: 96px;
  height: 25px;
}

Step 3:

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

Add:
# Tracker images
img_button_ticket_new = button_ticket_new.gif*25*96

Step 4:

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 5:

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 6:

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

A copy of the one from phpBB is here.

Step 7:

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