If all you need to do is create a negative version of pre-existing image, you can use ImageMagick on the command line:

$ converge in.png -negate out.png

I have IrfanView on Mac (via Wine project) and it works flawlessly. However, if you don't want to install Wine on your OSX, there are other alternatives, some mentioned here already. Personally, I like xnview - its free and probably includes everything you'd want (and supports making a negative image)

I solved my needs with https://www.xnview.com/en/xnviewmp/ and http://projects.apricus.be/en/xnconvert/ (looks like "negative" is not available in xnview, or I was unable to find it at least).

Thanks!

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!