Add a path to your environment CATgraphicPath.
example below is a server path for others to use the same icons.
CATGraphicPath=K:\CatiaApps\Standards\icons;C:\Program Files\Dassault Systemes\B23\win_b64\resources\graphic;C:
In the icons folder there should be a small and normal folder.
Small icon .bmp files need to be 17x17 pixel
Normal icon .bmp files can be 22x22 pixel
To make the icons show up first in the list you need to be clever while naming them I_###_icon_desc.bmp
example I_150_UDF_SHCS.bmp
Win 7
23SP5/24SP3, 3DVIA Composer 2015