The icon reference is in the liferay-portlet.xml for the ROOT application. Normally, if you want to change any of the things you find in the ROOT webapp, you are supposed to go through the EXT plugin route. In this case though, man, that seems like a lot of overhead to swap out an image. The referece to the image in the liferay-portlet.xml is
<icon>/html/icons/wiki.png</icon>
... I'd be inclined to go to /webapps/ROOT/html/icons/wiki.png with your image, unless you have an EXT plugin as part of your project already.
