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A map on the web works with “tiles”, so if you put directly one image, you have the same image tiled.

You should use something like mapwarper to produce a map-tiled version of your image
http://mapwarper.net/maps/37696#Preview_Rectified_Map_tab

And add this to uMap as a custom background (with proper zoom limits and bounds).
http://umap.openstreetmap.fr/en/map/custom-background_296199