Lastly, Irfanview doesn't write the coordinate system to the file nor copies over *.prj or *.xml files. This causes problems when adding to ArcMap. Use ArcCatalog, the options to define a Spatial Reference for a raster therein (preferably and easiest to import from the original file using the Import option).
- Right click new ECW file in ArcCatalog
- Choose Properties from context menu
- Choose Spatial Reference/Edit
- Choose Import
- Select original ECW file to import from
Now you should be able to add the new ECW file to ArcMap without issues.
*** UPDATE ***:
Hmmm, that was a bit optimistic ;(... When I tried it, the image does display, but if it is resampled to a smaller size, Irfanview/ArcMap doesn't register a bigger cell size, and just reduces the coverage of the image to a fraction of the original extent, which is of course completely wrong. E.g. if I choose to reduce size in Irfanview to 25% of the original, the extent in ArcMap comes out as 25% as well.
Of course, Irfanview was never designed to handle GIS data and be able to handle geographical coordinate system stuff.