The icons from the Cisco icon set are encoded in a way that is not supported by Java's default image framework.
As a workaround, open the Cisco icon you want to import in an image editor application like the GIMP or Adobe Photoshop and convert the icon into an image format that uses the default sRGB color model. E.g. opening the desired icon in the image editor aplication and saving it in PNG format (without any modifications) should result in an image that yEd can import.
The next version of yEd will feature improved image support (thanks to TwelveMonkeys) that can handle the Cisco icons as well.