(second from right)
This is used to create lists of images for use with the Action Manager.
To add images to the list double click the icon and use the Add, Delete and Clear buttons. To find suitable icons search the Program Files\Common Files directories or the ProgramFiles\Microsoft directories.

Here images 0-3 have been picked up from standard actions in the Action Manager while image 4 has been added by clicking on the Add button. In the Action Manager the image in the list can be selected for an action in a toolbar by setting the item and setting its ImageIndex to the one in the ImageList. In this example the Bold action has been changed to image 4 (not recommended!):

(Select the Format category in the Action Manager, click on Bold and change the ImageIndex property in the Object Inspector.)