From the wm reference page: [L1 ]:
Sets the titlebar icon for window, based on the named photo images. If -default is specified, this is applied to all future created toplevels as well. The data in the images is taken as a snapshot at the time of invocation. If the images are later changed, this is not reflected to the titlebar icons. Multiple images are accepted, to allow different images sizes (e.g., 16x16 and 32x32) to be provided. The window manager may scale provided icons to an appropriate size.