Well, there is nothing much special about those icons. It's more about your graphics skills... I usually export some actual icon from autocad and make just small adjustments in Window Paint program (with zoom of 400%). But it could be done with Gimp or some other graphics program or some programs specifically focused on icons... you may look for it -- maybe THIS ONE ? Even there is a small icon editor build-in inside of autocad (see the screenshot, "Edit" option above the red arrow)
Small icons are 16x16, large 32x32 px
Use *.bmp, newer versions of autocad supports *.png as well
Use color 192,192,192 which goes to be a transparent in autocad.
If nothing changed in new versions then large icons are unable to export from autocad - some bug or something.
