"image invisible" meaning in Français

{
  "categories": [
    {
      "kind": "other",
      "name": "Compositions en français",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Locutions nominales en français",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Français",
      "orig": "français",
      "parents": [],
      "source": "w"
    }
  ],
  "etymology_texts": [
    "Composé de image et de invisible."
  ],
  "forms": [
    {
      "form": "images invisibles",
      "tags": [
        "plural"
      ]
    }
  ],
  "lang": "Français",
  "lang_code": "fr",
  "pos": "noun",
  "pos_title": "Locution nominale",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en français de l’Internet",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Pixel espion"
      ],
      "id": "fr-image_invisible-fr-noun-b5YtlWWY",
      "raw_tags": [
        "Internet"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "\\i.ma.ʒ‿ɛ̃.vi.zibl\\"
    }
  ],
  "tags": [
    "feminine"
  ],
  "word": "image invisible"
}

{
  "categories": [
    "Compositions en français",
    "Locutions nominales en français",
    "français"
  ],
  "etymology_texts": [
    "Composé de image et de invisible."
  ],
  "forms": [
    {
      "form": "images invisibles",
      "tags": [
        "plural"
      ]
    }
  ],
  "lang": "Français",
  "lang_code": "fr",
  "pos": "noun",
  "pos_title": "Locution nominale",
  "senses": [
    {
      "categories": [
        "Lexique en français de l’Internet"
      ],
      "glosses": [
        "Pixel espion"
      ],
      "raw_tags": [
        "Internet"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "\\i.ma.ʒ‿ɛ̃.vi.zibl\\"
    }
  ],
  "tags": [
    "feminine"
  ],
  "word": "image invisible"
}