Thumbnail Image not showing up

Hello. I am developing a connect app. I added thumbnails to my app like so:

  "jiraDashboardItems": [
        "description": {
          "value": "...",
          "i18n": "...",
          "authentication": "..."

        "url": "...",
        "configurable": true,
        "thumbnailUrl": "/images/thumbnail.png",
        "refreshable": true,
        "name": {
          "value": "...",
          "i18n": "..."
        "key": "..."

I am able to see this thumbnail only in the current browser I used during development, but not in any other browser. My teammates cant see it either. Please help!