@Stratoula_Kalafateli is correct. There is no way to upload images to Elasticsearch to be used as a background and a planar coordinate system is actually more a hack of assuming it as geographic coordinates so they need to be in the domain of
[180,-180] for the X axis and
[90,-90] for the Y axis.
The blog post explains how to convert an image into a tile service that follows the tiled web map spec. You can use many Open Source software to do this, not only GeoServer.
Alternatively, if the layout of your factory is not too complicated and you don't have hard requirements in styling, is probably much easier for you to trace it into a vector dataset as a GeoJSON file and then upload it to Elasticsearch with Kibana to render it directly in Kibana Maps. Think of it like in a normal map replacing the standard basemap by the world countries dataset.
Hope this helps!