The best open source alternative to nomacs is ImageGlass. If that doesn't suit you, our users have ranked more than 50 alternatives to nomacs and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to nomacs are JPEGView (Free, Open Source), qView (Free, Open Source), qimgv (Free, Open Source) and Gwenview (Free, Open Source).
ImageGlass is a simple picture viewer with which you will be able to display your image collection in a very similar way to the one that Windows provides by default, but with some additional functions.
Eye of GNOME is the official image viewer for the GNOME Desktop environment. With it, you can view single image files, as well as large image collections. The Eye of GNOME supports a variety of image file formats.
PhotoQt is a simple, yet powerful and good looking image viewer, written in Qt, published as open-source, and completely free. Photo comes with a variety of features. Most features you would expect from a image viewer should be there.
QuickViewer is a graphic image viewer for comfortable browsing of many images. This software performs reading of image data in advance. And drawing the imported image data on the screen with OpenGL. You can browse images with much smoother response than ever before.