These can be located in several areas. The first place I look is in the /mod/forum folder for example. I believe that if Moodle does not find it anywhere else, this is where it looks.
The next place to look is in the current theme you are using. For example the Chameleon theme has its own set buried in its pix/mod folder. See Alternate icon set for more info about other icon locations in MoodleDocs.
The /pix folder has edit and other icons like that.
Hope this helps.