When you double click one of the images displayed in xnviewmp the image opens in a window and then under the 'Image' menu item, there is a submenu named 'Map'. Selecting the Map submenu opens a list of functions that can be performed on the image, and one of them is 'Negative'.

I only spent like five minutes with it and then I tried donwloading xconvert.

(I am part of the "read the docs only too late if ever" crew).

Thanks a lot!