To keep the look and feel of the Joomla! administrator panel consistent, it's best to use icons that are sized to 48 x 48 pixels.
If you now refresh the administration panel's dashboard, you'll see the new icon appear:
By changing the path to icon images in the mod_quickicons.php
file, you can change the appearance of the icons in the administrator panel.
As always, editing "core" Joomla! files (as in this case) may cause your changes to disappear when you upgrade to a newer version of Joomla!, so it's worthwhile maintaining a separate copy of all of the files you've just changed, just in case!