Cnfhbr has found out why the maps made with jnx2img were not displayed in Monterra.
I've changed the converter accordingly, and it seems that maps work correctly, at least in Monterra and GPSMAP 62.

The updated converter can be downloaded from the link in the first message of this thread.

Giomen, could you please check if the maps made with the new version of jnx2img show up correctly in chartplotter?