The OGo templates lookup icons by name. An attempt to describe some of them ;-)

Icons used in the dock (eg blue theme)

menu_news_labeled.gif
menu_persons_labeled.gif
menu_enterprises_labeled.gif
menu_projects_labeled.gif
menu_scheduler_labeled.gif
menu_jobs_labeled.gif
menu_bookmarks_labeled.gif

The email dock icon can have several different states:

menu_email_labeled_inactive.gif
menu_email_labeled.gif
menu_email_new_labeled.gif

For "docked" projects OGo uses the "bookmarks" icon:

menu_bookmarks_labeled.gif

Filetype icons

The names for filetype icons, like MIME parts in the webmailer or files in the document storage, are calculated using the MIME type.
OGo will first look for mimeicon_MIMEMAJOR_MIMEMINOR_16x19.gif where MIMEMAJOR is something like "image" and MIMEMINOR is something like "gif", eg mimeicon_image_gif_16x19.gif.
If OGo does not find an exact match, it will attempt to find mimeicon_MIMEMAJOR_16x19.gif.
Finally, if this also fails, OGo will show the question mark icon (which icon is that?).

...
mimeicon_text_16x19.gif
mimeicon_text_css_16x19.gif
mimeicon_text_html_16x19.gif
mimeicon_text_plain_16x19.gif
mimeicon_text_rfc822_headers_16x10.gif
...

Todo: find out whether "document_xxx.gif" is still used somewhere

Appointment Type Icons

Todo: describe (apt_10x10.gif and apt_icon_TYPE.gif).

...
apt_icon_birthday.gif
apt_icon_call.gif
apt_icon_default.gif
apt_icon_duedate.gif
...

Tabs

Tab frames are also constructed using images and support a specific style where an unselected left tab can look different from the other.
In the OGo theme the icons have a size of 101x22 pixels, not sure whether this is required. Note that the tabs are used as background images.
Note: I think the icons must have a size of exactly 100x22 pixels.

tab_.gif
tab_selected.gif
tab_left.gif
corner_left.gif
corner_right.gif

corner_left.gif is only active if the leftmost tab is not selected. The same goes for tab_left.gif which is the icon of an unselected left tab.

The OGo blue or orange themes use that feature.