When you include icons, take a few guidelines into consideration:

• For contributions: Save the icons in your module's directory
• For core: Save the icons in the /misc directory
• Use Tango icons or Lullabot Icons whenever possible

If no icons exist, refer to the icon style guidelines of Tango. There is also a Lullabot article on making icons.

Note that Drupal.org cannot host Tango icons in git due to licensing problems. You will have to download and install the icons yourself from Tango's site, and comply with their licensing. However, Lullabot icons are GPL so you can use those in git.