The Java UI plugin provides the Interface ISharedImages to access standard images which can be used in own plugins.
e.g.
Image image = PlatformUI.getWorkbench().getSharedImages().getImage(ISharedImages.IMG_OBJ_ELEMENT);
This works very well but I did not find an overview of the available images. Can someone please point me to such a documentation?
Some of them you can find here, but keep in mind that other 3rd party plugins can add another set of shared images for which, you would have to refer to that plugin documentation.