G'MIC is an open and full-featured framework for image processing in Gimp, Paint.Net, and Krita.
You have several different user interfaces in G'MIC that will convert/manipulate/filter/visualize generic image datasets, from 1d scalar signals to 3d+t sequences of multi-spectral volumetric images.
XnView Full is a powerful tool that enables users to view, process, and convert image files easily. XnView Full has all plug-ins and languages included. XnView Standard has just a few add-ons included.