Each portlet can be represented by a unique icon that you can see in the portlet registry or page editor. This icon can be changed by adding an image to the directory of the portlet webapplication:
skin/DefaultSkin/portletIcons/icon_name.png.
To use the icon correctly, it must be named after the portlet.
For example, the icon for an account portlet named AccountPortlet would be located at:
skin/DefaultSkin/portletIcons/AccountPortlet.png
You must use skin/DefaultSkin/portletIcons/ for the directory to store the portlet icon regardless of what skin is going to be used.