Each Moodle theme can either use the standard icon set located in moodle/pix directory & subdirectories OR its own icon set located in moodle/theme/"your_theme"/pix/ directory & subdirectories.
The choice is made in your theme's config.php file, line:
$THEME->custompix = true;
Erin, to know which is which you have to find out which theme your institution is using, whether it is a unique theme site-wide or teachers are allowed to choose their preferred theme for their own courses.
Hope that helps,
Joseph
PS If the little green guy icon looks like the one below, then it means your institution have chosen the use the NUVOLA ICON THEME set. See this discussion about it.
