From phpBB Development Wiki

This file contains the configuration information for the style's imageset
There are at least two imageset.cfg files required for each phpBB3 style.
The main imageset.cfg file is located in /styles/<style_name>/imageset/imageset.cfg
The language imageset.cfg file is located in /styles/<style_name>/imageset/<lang>/imageset.cfg
Each language installed must have it’s own language imageset.cfg file as well to function properly.

The individual lines break down like so:
Example: img_site_logo = site_logo.gif*52*139
img_site_logo is the name of the template variable.
site_logo.gif is the file name of the image.
*52*139 is the dimensions of the image begining with the height first, height is 52px and width is 139px.

The file contains the following information for use within phpBB3, each file requires the standard header:

name = NameofStyle
copyright = © 2007 Name of Creator
version = 1.2.3

Version numbers usually follow the typical Version Numbering guidelines.

As well as the following values.
If a style does not use a specific image, that value may remain blank. (see img_icon_friend and img_icon_foe below).

imageset.cfg Example File located in /imageset/

# General Information about this style
name = mystyle
copyright = © myself, 2007
version = 1.2.3

# Images
img_site_logo = site_logo.gif*52*139
img_poll_left = poll_left.gif*10*10
img_poll_center = poll_center.gif*10*10
img_poll_right = poll_right.gif*10*10
img_icon_friend = 
img_icon_foe = 

img_forum_link = forum_link.gif*27*27
img_forum_read = forum_read.gif*27*27
img_forum_read_locked = forum_read_locked.gif*27*27
img_forum_read_subforum = forum_read_subforum.gif*27*27
img_forum_unread = forum_unread.gif*27*27
img_forum_unread_locked = forum_unread_locked.gif*27*27
img_forum_unread_subforum = forum_unread_subforum.gif*27*27

img_topic_moved = topic_moved.gif*27*27

img_topic_read = topic_read.gif*27*27
img_topic_read_mine = topic_read_mine.gif*27*27
img_topic_read_hot = topic_read_hot.gif*27*27
img_topic_read_hot_mine = topic_read_hot_mine.gif*27*27
img_topic_read_locked = topic_read_locked.gif*27*27
img_topic_read_locked_mine = topic_read_locked_mine.gif*27*27

img_topic_unread = topic_unread.gif*27*27
img_topic_unread_mine = topic_unread_mine.gif*27*27
img_topic_unread_hot = topic_unread_hot.gif*27*27
img_topic_unread_hot_mine = topic_unread_hot_mine.gif*27*27
img_topic_unread_locked = topic_unread_locked.gif*27*27
img_topic_unread_locked_mine = topic_unread_locked_mine.gif*27*27

img_sticky_read = sticky_read.gif*27*27
img_sticky_read_mine = sticky_read_mine.gif*27*27
img_sticky_read_locked = sticky_read_locked.gif*27*27
img_sticky_read_locked_mine = sticky_read_locked_mine.gif*27*27
img_sticky_unread = sticky_unread.gif*27*27
img_sticky_unread_mine = sticky_unread_mine.gif*27*27
img_sticky_unread_locked = sticky_unread_locked.gif*27*27
img_sticky_unread_locked_mine = sticky_unread_locked_mine.gif*27*27

img_announce_read = announce_read.gif*27*27
img_announce_read_mine = announce_read_mine.gif*27*27
img_announce_read_locked = announce_read_locked.gif*27*27
img_announce_read_locked_mine = announce_read_locked_mine.gif*27*27
img_announce_unread = announce_unread.gif*27*27
img_announce_unread_mine = announce_unread_mine.gif*27*27
img_announce_unread_locked = announce_unread_locked.gif*27*27
img_announce_unread_locked_mine = announce_unread_locked_mine.gif*27*27

img_global_read = announce_read.gif*27*27
img_global_read_mine = announce_read_mine.gif*27*27
img_global_read_locked = announce_read_locked.gif*27*27
img_global_read_locked_mine = announce_read_locked_mine.gif*27*27
img_global_unread = announce_unread.gif*27*27
img_global_unread_mine = announce_unread_mine.gif*27*27
img_global_unread_locked = announce_unread_locked.gif*27*27
img_global_unread_locked_mine = announce_unread_locked_mine.gif*27*27

img_pm_read = topic_read.gif*27*27
img_pm_unread = topic_unread.gif*27*27

img_icon_back_top = icon_back_top.gif*11*11

img_icon_contact_aim = icon_contact_aim.gif*20*20
img_icon_contact_email = icon_contact_email.gif*20*20
img_icon_contact_icq = icon_contact_icq.gif*20*20
img_icon_contact_jabber = icon_contact_jabber.gif*20*20
img_icon_contact_msnm = icon_contact_msnm.gif*20*20

img_icon_contact_www = icon_contact_www.gif*20*20
img_icon_contact_yahoo = icon_contact_yahoo.gif*20*20

img_icon_post_delete = icon_post_delete.gif*20*20

img_icon_post_info = icon_post_info.gif*20*20

img_icon_post_report = icon_post_report.gif*20*20
img_icon_post_target = icon_post_target.gif*9*11
img_icon_post_target_unread = icon_post_target_unread.gif*9*11

img_icon_topic_attach = icon_topic_attach.gif*10*7
img_icon_topic_latest = icon_topic_latest.gif*9*11
img_icon_topic_newest = icon_topic_newest.gif*9*11
img_icon_topic_reported = icon_topic_reported.gif*14*16
img_icon_topic_unapproved = icon_topic_unapproved.gif*14*16

img_icon_user_profile = icon_user_profile.gif*11*11

img_icon_user_warn = icon_user_warn.gif*20*20

imageset.cfg example file located in /imageset/<lang>/

The individual language imageset.cfg files do not need to contain any copyright information.

# Images
img_icon_contact_pm = icon_contact_pm.gif*20*28

img_icon_post_edit = icon_post_edit.gif*20*42
img_icon_post_quote = icon_post_quote.gif*20*54

img_icon_user_online = icon_user_online.gif*58*58
img_icon_user_offline = icon_user_offline.gif
img_icon_user_search = icon_user_search.gif

img_button_pm_forward = button_pm_forward.gif*25*96
img_button_pm_new = button_pm_new.gif*25*84
img_button_pm_reply = button_pm_reply.gif*25*96

img_button_topic_locked = button_topic_locked.gif*25*88
img_button_topic_new = button_topic_new.gif*25*96
img_button_topic_reply = button_topic_reply.gif*25*96

Images