You can place a fallback image under the WebContent/skin/images folder of your portlet application that designer will use to draw the image during design-time.
Actually, the way the runtime resolves the skin image is that it first checks if the current skin contains the requested image. If the skin has the image then it will use it. If the skin doesn’t have the image, it will fallback to an image at the same path inside your portlet application. So it is a good idea to always have a copy of the image in your portlet application as a fallback just in case.
I don’t believe there is a documented list of images in the provided skins. You could look at the Images tab in the skin editor portlet to see what images are available for a specific skin.
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