These are gif icons.

They are installed in $(GISBASE)/etc/gui/icons

They follow the following naming scheme:
[class]-[member].gif

The following classes are being used:

element

These are icons for grass database elements such as cell, rast3, vector, windows, etc. and for symbol, the other type of thing that the tcl GSelect procedure handles.

module

Members of module are programs or parts of programs. For example module-d.nviz.gif. The program name can consist of just the first few . seperated parts of the name. For example module-r.gif could be used for all raster programs. The last . must be removed; module-v.in..gif won't match anything.

gui

Icons used by gui.tcl.
gui-cmd.gif is needed for one version of gui.tcl

file

Icons for file operations like open, close, etc.
file-open.gif is needed for one version of gui.tcl

edit

Icons for edit operations like cut, copy, paste, etc.
edit-copy.gif is needed for one version of gui.tcl

status

Status icons for grass. Any lowercase key to GRASS_INFO_*, like warning, error, message. Also success and failure for exit codes.

channel

Icons for channels of images