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The gadget thumbnails are displayed in the Page Editor window when you edit a page. The thumbnail image size needs to be consistent for all gadgets in the list. The current size (in eXo Platform) is 80 x 80 px, so you should select an image of this size in PNG (preferred), JPG or GIF format for your gadget thumbnail.
The image can also be one from a public website (absolute URL), for example,
thumbnail="http://www.example.com/images/HelloWorld-icon.jpg"/or from an internal image (relative URL):thumbnail="image/HelloWorld-icon 80x80.png"/. Add that attribute to the <ModulePrefs> tag in theHelloGadget.xmlfile.