Edit in JSFiddle

let bounds = [
  [0, 0],
  [847, 1280]
];

let map = L.map('map', {
  crs: L.CRS.Simple,
  maxZoom: 0,
  minZoom: -4,
  maxBounds: bounds
});

let image = L.imageOverlay('http://kody.wig.pl/wp-content/uploads/factory_UML.png', bounds).addTo(map);

map.fitBounds(bounds);

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.5.1/leaflet.css" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/leaflet/1.5.1/leaflet.js"></script>

<div id="map"></div>

html,
body {
  height: 100%;
  margin: 0;
}

#map {
  width: 100%;
  height: 100%;
}